136 GEOLOGICAL BIOLOGY. 



€nce upon the Glossophora, as well as upon other animals. The study of their 

 conditions has a particular interest to the paleontologist, since he is thus able to 

 account for the conditions under which the fossils lived, and the mode of forma- 

 tion of marine sediments. One knows, in a general way, that the temperature 

 ■of the ocean goes on diminishing from the surface to the bottom, and that it 

 attains a temperature approximately constant of 4° to 5" Cent, at the depth of 

 500 feet; it descends scarcely to zero (32° F.) at great depths ; the conditions of 

 submarine existence are thus approximately constant in abysmal regions, while 

 they present the greatest range of variation in the shore regions of slight depth 

 in the tropics. 



The bathymetric distribution of Mollusca was studied in 1S30 by Andouin and 

 Milne Edwards ; and later, upon new data, by Sars in Norway (1835) and Ed. 

 Forbes in the .^gean Sea and in England. The most important results in this 

 direction have been attained by the expeditions of the Porcupine (1869-70), of 

 the Challenger (1873-76), of the Gazelle (1874-76), of the Tuscarora (1874-76), 

 of the Blake (1877-78), of the Voraigen (1S76-7S), of the Voraillem (1880). 



The Zonal Distribution of the Ctenobranchina. — Restricting our 

 attention to the families of CtcnobrancJiina, and using for the 

 purpose the classification into families of F. Barnard,* which 

 are 44, we are able to see some evidence of the particular 

 connection between form and bathymetric distribution. Of 

 these families three have land species, and two of the fami- 

 lies are restricted to a land habitat (Cyclophorids and Cyclo- 

 stomidae). There are five families of Avhich the species are 

 all fresh-water species (Paludinidae, Ampullaridre, Bithyniidae, 

 Valvatidae, and Melaniida;). One family, Hydrobiidse, has 

 both littoral and brackish water species. The remaining 

 thirty-four families are all marine; of them many of the lit- 

 toral species are able to endure exposure to the air and some 

 contamination of the water, but the normal habitat of all is 

 marine. Some of the families are limited in downward dis- 

 tribution : such are the families Truncatellidse, Hydrobiidae, 

 Janthinidae (a pelagic type), Cypraeidae, Solariidae, Purpuridae, 

 and Terebridse. Others reach downward to the abysmal depths, 

 as Littorinidae, Rissoidae, Cerithiidae, Naticidae, Scalaridae, 

 PyramidellidjE, Eulimidai, Muricidae, Pleurotomids ; and it 

 is interesting to note that of these families, having a bathy- 

 metric distribution from the abysmal depth to the littoral 

 zone, several are also the most ancient in geological range ; 

 the Littorinidae, the Naticidae, and the Pyramidellidae are re- 

 ported from as early as the Silurian era. The second section, 



* " Elements de Paleontologie," 1893. 



