62 Waltker — Origin and Peopling of the Deep Sea. 



still live. Therefore we must also point out that in the 

 present shallow sea there actually live a number of uncom- 

 monly long enduring Paleozoic genera : 



Of brachiopods, Lingula, Rhynchonetta 



Of bivalves, Area, Avicula, Astarte, Leda, Mytihts 



Of univalves, Capufots, PleurotO)naria 



Of cephalopoda, Nautilus 



Of worms, Serpula 



Of starfishes, Astropeeten. 



Limidus, the last representative of Silurian horseshoe crabs, 

 is a coast dweller, and Geratodus, rooted in the Devonian, still 

 lives in Australian rivers. 



We must thereto also add a number of forms without skele- 

 tons which are phylogenetically very old and which must have 

 had their origin in the pre-Cambrian faunas. Hydra and 

 Amphioxus as well as the Asconia sponges, Planarians and 

 Holothurians are mostly dwellers in very shallow water, and 

 all these forms reach back into the oldest part of the earth's 

 history. 



Only the Cambrian genus Discina, some Silurian bivalves 

 such as Area and JVucula, univalves such as Dentalium. and 

 the Devonian Terebratida, have descended into the deep sea, 

 but it is naturally very easy to suppose that they first began 

 this migration at a later time. Mustering now the remaining 

 animals with skeletons living below 2000 meters and with a 

 clear understanding as to their paleontologic position, there can 

 be no doubt that the oldest genera date from the Triassic and 

 Jurassic periods. 



The Euretidse among the hexactinellid siliceous sponges, the 

 turbinolids among the corals, Pentacrinus among the crinoids, 

 Ophioglypha and Asterias among the starfishes, Echinus 

 among the sea urchins and Penwus among the crabs are forms 

 whose oldest kin belong to the Mesozoic age. Their migra- 

 tion into the deep sea can therefore at best date from the Tri- 

 assic. 



Very close is the relationship of the present deep sea fauna 

 to the animal world of the Jurassic and Cretaceous periods. 

 The German Valdivia expedition found the nearest kin of the 

 upper Jurassic Eryon in the characteristic deep sea crabs 

 Pentacheles, Willemmsia and Polycheles, and A. Agassiz has 

 shown that most of the deep sea urchins are related to Creta- 

 ceous genera. 



The deep sea corals belong almost entirely to Cretaceous gen- 

 era. It is further noteworthy that typical Tertiary forms are 

 very rare in the deep sea. The migration must therefore in 

 general have ceased in the Tertiary. 



Were the present deep sea fauna laid before a paleontologist 

 " without designation of locality," he would on the basis of 



