In the early days of deep-sea exploration, when naturalists were becoming aware of the rich fauna inhabiting the abysses of the ocean, which till then had been supposed to be barren of all life, it was confidently expected that representatives would be discovered of some of the animals known as fossils from the earlier geological periods. It was believed that the great ocean basins had remained unchanged for vast periods of geological time, and that numerous "living fossils" would be found surviving in the depths. These hopes have not been fully realized, for the deep-sea fauna as a whole has proved to be of a comparatively modern type; nevertheless, it does include a considerable number of primitive and old-fashioned forms of life, some of which belong to groups elsewhere extinct. This is conspicuously the case among the Crustacea. The lobster-like Eryonidea, which at the present day are only found in the deep sea, were long known as fossils before they were discovered to survive as living animals. The existing species (Fig. 46) are all blind, with only vestiges of eye-stalks, and they may be readily distinguished by the fact that the first four, and sometimes all five, pairs of legs end in chelæ, no other Decapods having more than three pairs of chelate legs. The fossils occur in rocks of the Secondary Period, from the Trias to the early Cretaceous. Some of them, at least, had well-developed eyes, and probably lived in shallow water. This was almost certainly the habitat of those (Fig. 47) that are found preserved in a marvellously perfect state in the lithographic limestone of Solenhofen (famous for the discovery of _Archæopteryx_ and many other remarkable fossils), which is believed to have been deposited in a lagoon. After the early part of the Cretaceous epoch, the Eryonidea are no longer found as fossils, and it is, at all events, a probable conjecture that about that period they forsook the shallow waters for the deeper recesses of the ocean, where their descendants have held their own till the present day. [Illustration: FIG. 46--_Polycheles phosphorus_, ONE OF THE ERYONIDEA, FEMALE, FROM THE INDIAN SEAS. (From British Museum Guide, after Alcock.)] [Illustration: FIG. 47--_Eryon propinquus_, ONE OF THE FOSSIL ERYONIDEA, FROM THE JURASSIC ROCKS OF SOLENHOFEN. (From Lankester's "Treatise on Zoology," after Oppel.)] Another group of deep-sea Crustacea which has affinities with certain fossil forms is the little family Homolodromiidæ among the Crabs. It has already been mentioned that the Dromiacea are the most primitive tribe of the Brachyura, and Professor Bouvier has shown that among these the Homolodromiidæ approach most nearly to the lobster-like forms from which the Crabs have been derived. He has further shown that the members of this family closely resemble in the arrangement of the grooves upon the carapace the extinct Prosoponidæ, which are known as fossils from Jurassic and Cretaceous rocks.