Its head and mouth are more similar in appearance to those of a hippopotamus than to any other earthly animal, except that from the sides of the lower jawbone two mighty horns curve slightly downward toward the front.

Its two huge eyes inspired my greatest curiosity. They extend in two vast, oval patches from the center of the top of the cranium down either side of the head to below the roots of the horns, so that these weapons really grow out from the lower part of the eyes, which are composed of several thousand ocelli each.

This eye structure seemed remarkable in a beast whose haunts were upon a glaring field of ice and snow, and though I found upon minute examination of several that we killed that each ocellus is furnished with its own lid, and that the animal can at will close as many of the facets of his huge eyes as he chooses, yet I was positive that nature had thus equipped him because much of his life was to be spent in dark, subterranean recesses.

Shortly after this we came upon the hugest apt that we had seen. The creature stood fully eight feet at the shoulder, and was so sleek and clean and glossy that I could have sworn that he had but recently been groomed.

He stood head-on eyeing us as we approached him, for we had found it a waste of time to attempt to escape the perpetual bestial rage which seems to possess these demon creatures, who rove the dismal north attacking every living thing that comes within the scope of their far-seeing eyes.

Even when their bellies are full and they can eat no more, they kill purely for the pleasure which they derive from taking life, and so when this particular apt failed to charge us, and instead wheeled and trotted away as we neared him, I should have been greatly surprised had I not chanced to glimpse the sheen of a golden collar about its neck.

Thuvan Dihn saw it, too, and it carried the same message of hope to us both. Only man could have placed that collar there, and as no race of Martians of which we knew aught ever had attempted to domesticate the ferocious apt, he must belong to a people of the north of whose very existence we were ignorant-possibly to the fabled yellow men of Barsoom; that once powerful race which was supposed to be extinct, though sometimes, by theorists, thought still to exist in the frozen north.

Simultaneously we started upon the trail of the great beast. Woola was quickly made to understand our desires, so that it was unnecessary to attempt to keep in sight of the animal whose swift flight over the rough ground soon put him beyond our vision.

For the better part of two hours the trail paralleled the barrier, and then suddenly turned toward it through the roughest and seemingly most impassable country I ever had beheld.

Enormous granite boulders blocked the way on every hand; deep rifts in the ice threatened to engulf us at the least misstep; and from the north a slight breeze wafted to our nostrils an unspeakable stench that almost choked us.

For another two hours we were occupied in traversing a few hundred yards to the foot of the barrier.

Then, turning about the corner of a wall-like outcropping of granite, we came upon a smooth area of two or three acres before the base of the towering pile of ice and rock that had baffled us for days, and before us beheld the dark and cavernous mouth of a cave.

From this repelling portal the horrid stench was emanating, and as Thuvan Dihn espied the place he halted with an exclamation of profound astonishment.

"By all my ancestors!" he ejaculated. "That I should have lived to witness the reality of the fabled Carrion Caves! If these indeed be they, we have found a way beyond the ice-barrier.

"The ancient chronicles of the first historians of Barsoom-so ancient that we have for ages considered them mythology-record the passing of the yellow men from the ravages of the green hordes that overran Barsoom as the drying up of the great oceans drove the dominant races from their strongholds.

"They tell of the wanderings of the remnants of this once powerful race, harassed at every step, until at last they found a way through the ice-barrier of the north to a fertile valley at the pole.

"At the opening to the subterranean passage that led to their haven of refuge a mighty battle was fought in which the yellow men were victorious, and within the caves that gave ingress to their new home they piled the bodies of the dead, both yellow and green, that the stench might warn away their enemies from further pursuit.

"And ever since that long-gone day have the dead of this fabled land been carried to the Carrion Caves, that in death and decay they might serve their country and warn away invading enemies. Here, too, is brought, so the fable runs, all the waste stuff of the nation-everything that is subject to rot, and that can add to the foul stench that assails our nostrils.

"And death lurks at every step among rotting dead, for here the fierce apts lair, adding to the putrid accumulation with the fragments of their own prey which they cannot devour. It is a horrid avenue to our goal, but it is the only one."

"You are sure, then, that we have found the way to the land of the yellow men?" I cried.

"As sure as may be," he replied; "having only ancient legend to support my belief. But see how closely, so far, each detail tallies with the world-old story of the hegira of the yellow race. Yes, I am sure that we have discovered the way to their ancient hiding place."

"If it be true, and let us pray that such may be the case," I said, "then here may we solve the mystery of the disappearance of Tardos Mors, Jeddak of Helium, and Mors Kajak, his son, for no other spot upon Barsoom has remained unexplored by the many expeditions and the countless spies that have been searching for them for nearly two years. The last word that came from them was that they sought Carthoris, my own brave son, beyond the ice-barrier."

As we talked we had been approaching the entrance to the cave, and as we crossed the threshold I ceased to wonder that the ancient green enemies of the yellow men had been halted by the horrors of that awful way.

The bones of dead men lay man high upon the broad floor of the first cave, and over all was a putrid mush of decaying flesh, through which the apts had beaten a hideous trail toward the entrance to the second cave beyond.

The roof of this first apartment was low, like all that we traversed subsequently, so that the foul odors were confined and condensed to such an extent that they seemed to possess tangible substance. One was almost tempted to draw his short-sword and hew his way through in search of pure air beyond.

"Can man breathe this polluted air and live?" asked Thuvan Dihn, choking.

"Not for long, I imagine," I replied; "so let us make haste. I will go first, and you bring up the rear, with Woola between. Come," and with the words I dashed forward, across the fetid mass of putrefaction.

It was not until we had passed through seven caves of different sizes and varying but little in the power and quality of their stenches that we met with any physical opposition. Then, within the eighth cave, we came upon a lair of apts.

A full score of the mighty beasts were disposed about the chamber. Some were sleeping, while others tore at the fresh-killed carcasses of new-brought prey, or fought among themselves in their love-making.

Here in the dim light of their subterranean home the value of their great eyes was apparent, for these inner caves are shrouded in perpetual gloom that is but little less than utter darkness.

To attempt to pass through the midst of that fierce herd seemed, even to me, the height of folly, and so I proposed to Thuvan Dihn that he return to the outer world with Woola, that the two might find their way to civilization and come again with a sufficient force to overcome not only the apts, but any further obstacles that might lie between us and our goal.