86 



MANUAL OF THE MOLLUSCA. 



Modiola Carolinensis. 



„ plicatula. M. Y. 

 Pinna muricata. SC. 

 Area ponderosa. SC. 

 „ pexata. M.F. 

 „ incongrua. SC. 

 „ transversa. M. Y. 

 Solemya velum. M. Y. 



„ borealis. M. 

 Cardium ventrieosum. SC. 



„ Mortoni. M. Y. 

 Lucina contracta. Y. 

 Astarte Mortoni. Y. 



„ bilunulata. F. 

 Cardita incrassata. F. 

 Venus mercenaiia. M. SC. 

 „ Mortoni. SC. F. 

 „ genama. M. Y. 

 Artemis discus. SC. 

 Petricola dactylus. M. SC. 

 „ pholadiformis. Y. 



Mactra similis. SC. M. 



„ solidissima. M. Y. 



„ lateralis. M. Y. 

 Lutraria lineata. F. 



„ canaliculata. Y. F. 

 Mesodesma arctata. M. Y. 

 Tellina tenta. M. SC. 



„ 8 sp. SC. F. 

 Semele aequalis. SC. 

 Cumingia tellinoides. M. 

 Donax fossar. Y. 



„ variabilis. G. F. 

 Solecurtus fragilis. M. SC. 

 caribbeeus. M. F. 

 Corbula contracta. M. F. 

 Periploma Leana. M. Y. 



„ papyracea, M. Y. 

 Lyonsia hyalina. Y. 

 Pandora trilineata. M. F. 

 Pholas costata. SC. F. 



„ semicostata. SC. 



LAND EEGIONS. 



Distribution of Land and Fresh-water Shells. 



The boundaries of the Natural-history land-regions are more 

 distinctly marked, and have been more fully investigated, than 

 their counterparts in the sea. Almost every large island has its 

 own fauna and flora ; almost every river system its peculiar 

 fresh-water fish and shells ; and mountain-chains like the Andes 

 appear to present impassable barriers to the "nations" of 

 animals and plants of either side. Exceptions, however, occur 

 which show that beyond this first generalisation there exists a 

 higher law. The British Channel is not a barrier between two 

 provinces, nor is the Mediterranean ; and the desert of Sahara 

 separates only two portions of the same zoological region. In 

 these and other similar instances the " barrier" is of later date 

 than the surrounding fauna and flora. 



It has been often remarked that the northern part of the map 

 of the world presents the appearance of vastly-extended, conti- 

 nental plains, much of which is, geologically speaking, new 

 land. In the southern hemisphere the continents taper oflT into 

 promontories and peninsulas, or have long since broken up into 

 islands. Connected with this is the remarkable fact that only 

 around the shores of the Arctic Sea are the same animals and 

 plants found through every meridian ; jmd that in passing south- 

 ward, along the three principal lines of land, specific identities 



