GEOGEAPHICAL DISTEIBUTION OF PLANTS AND ANIMALS. 



59 



31. 



degrees of light or darkness. Of reef -making Corals, or those that grow in plantations, 

 an account has already been given. 



Echinoderms are solely marine species, and they are found at all depths and tempera- 

 tures. Crinoids of the genus Pentacrinus (Fig. .30, page 58), and allied to Liassic kinds, 

 with species of Khizocrinus, Bathycrinus, etc., live at depths above 100 fathoms to below 

 1000, many where the temperature is below 40° F. Sea-urchius (Echinids) of the Cidaris, 

 Diadema, and Ananchytes families, related to Cretaceous types, occur at similar cold 

 depths. A. Agassiz states that the deep-sea fauna of the West Indies includes 5 Jurassic 

 genera of Echinids, 10 Cretaceous, 24 early Tertiary, and 4 of the later Tertiary. 



Brachiopods of the Tei'ebratulid type, much like Oolitic and Cretaceous forms, occur at 

 all depths, down to 18,000 feet ; and Discina, from the surface to cold depths exceeding 

 12,000 feet, but the most below 3000 feet ; Crania, at 600 to 1200 feet. Lingula occurs in 

 shallow waters. Species of the genera Atretia, Discina, and Waldheimia and others occur 

 beneath the Gulf Stream at depths of 9000 to 9600 feet. 



Under Mollusks : Pteropods are pelagic species, and live mostly near the surface. 

 Their shells occur in large numbers in the bottom deposit at depths mostly from 500 to 

 1500 fathoms in the "West Indies and some parts of the Pacific. 

 The form in Fig. 31 of a Mexican Gulf and Atlantic species 

 is much like that of many ancient Pteropods. Deep-sea 

 Gastropods are usually small. The genus Pleurotomaria has 

 only four living species known ; and P. Adansoniana lives at 

 a depth of 1200 feet. Trigonia is a shallow-water genus. The 

 Xautilus, the last of the Cephalopods having external shells, 

 is restricted to tropical and sub-tropical seas. 



Among Worms, the Serpulidse occur at great depths, 

 species having been obtained by the " Challenger" at depths 

 of nearly 18,000 feet. 



Some of the abyssal species of Crustaceans have been 

 shown to range from pole to pole. The large spiny crabs 

 of the genus Lithodesare probably among them. One of the 

 species, L. Agassizii, from a depth of 1000 fathoms under- 

 neath the Gulf Stream, is reported by Verrill (1884) as over 

 three feet hroad. Many of the deep-sea Crustaceans, accord- 

 ing to S. I. Smith, are remarkable for the large size of their 

 eggs. In some of the Eupaguri (Soldier-crabs), the eggs are 

 8 times the usual size (volume). 



The only surviving species of the Trilobite and Eurypterid 

 line are two of the genus Limulus, — one in eastern North 

 America, and the other in the China seas. Crustaceans are 

 found mostly at depths less than 3000 feet ; 2 only out of 

 100 Brachyurans dredged off the United States were from 

 depths greater than 3000 feet ; but 30 out of 60 Macrurans 

 were from greater depths, 13 of them from below 6000 feet, 

 and some at depths of 12,000 feet ; and one gigantic blind 

 species, Phoherus ccecus, is over 2 feet long. One Isopod, 

 Bathynomus giganteus, occurs eleven inches long; in com- 

 pensation for dark depths it has compound eyes comprising 

 4000 facets CMUne-Edwards) . 



Fishes. — The existing Ganoids — Sturgeons included — 

 live only in fresh waters, and are confined to America, Africa, 



and Australia. North America has 3 species of the genus Lepidosteus, and Africa 2 of 

 Polypterus ; and of the related Dipnoi, which are, as the name implies, two-way breathers, 

 they having lungs as well as gills, Queensland, northern Australia, has 2 species of Cera- 



Pteropod, genus Styliola. 

 A. Agassiz. 



X 5. 



