86 



MAIfiril OF THE MOLLtrSCA. 



Modiola Caf olinensis. 



Mactra similia. SC. M. 



„ plicatula. M. Y. 



„ solidissima. M. Y. 



Pinna muricata. SC. 



„ lateralis. M. Y. 



Area ponderosa. SC. 



liUtraria lineata. F. 



„ pexata. M.F. 



„ canaliculata. Y. T?. 



„ ineongrtta. SC. 



Mesodesma arctata. M. Y. 



„ transversa. M. Y» 



Tellina tenta. M. SC. 



Solemya velum. M. Y. 



„ 8 Sp. SC. F. 



„ borealis. M. 



Semele sequalis. SC. 



Cardium ventricosum. SC. 



Ciuningia tellinoides. M. 



„ Mortoni. M. Y. 



Donax fossar. Y. 



Lucina contracta. Y. 



„ variabilis. Gr. F. 



Astarte Mortoni. Y. 



Solecurtus fragilis. M. SC. 



„ bilunnlata. F. 



„ caribbffius. M. F. 



Cardita incrassata. F. 



CorbUla contracta. M. F. 



Venus mercenaria. M. SC. 



Periploma Leana. M. Y. 



„ Mortoni. SC. F. 



„ papjTacea. M. Y 



„ gemma. M. Y. 



Lyonsia hyalina. Y. 



Artemis discus, SC. 



Pandora trilineata. M. F. 



Petricola dactylus. M. SC. 



P'holas costata. SC. F. 



„ pholadiformisi Y» 



„ semicostata. SC. 



LAND BEGIONS. 



Distribution of Land and Fresh-water Shells. 



Tlie boundaries of tlie Natural-history land-regions are more 

 distinctly marked, and have been more fully investigated, tban 

 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 eadsts 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 off 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 ; Jtnd that in passing south- 

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



