86 MANUAL OF THE MOLLUSCA. 
Modiola Carolinensis. Mactra similis. SC. M. 
5) plicatula. «Mi. Y. - » solidissima. M. Y. 
Pinna rouricata. SC. »  Jateralis, M. Y. 
Arca ponderosa. SC. Lutraria lineata. F. 
» pexata. M.F. 53 canajiculata. Y. FF. 
» inconygrua. SC. Mesodesma arctata. M. Y. 
», transverse. M. Y. Tellina tenta. M. SC. 
Solemyavelum. M. Y. Ss ROISD> oC. ke 
» borealis. M. Semele zyualis. SC. 
Cardium ventricosum. SC. Cumingia tellinoides. M. 
a Mortoni. M. Y. Donax fossar. Y. 
Lucina contracta. Y. 3 variabilis. G. F. 
Astarte Mortoni. Y. Solecurtus fragilis. M. SC. 
»,  Dilunulata. F. ae caribbeus. M. F. 
Carditaincrassata. F. Corbula contracta. M. F. 
Venus mercenaria. M. SC. Periploma Leana. M. Y. 
» Mortoni. SC. F. 9 papyracea. M. Y. 
Sseeeerelmina. VL. ps Lyonsia hyalina. Y. 
Artemis discus. SC. Pandora trilineata. M. F. 
Petricola dactylus. M. SC. Pholas costata. SC. F. 
+5 pholadiformis. Y. »  semicostata. SC. 
LAND REGIONS. 
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 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 ; sud that in passing south- 
ward, along the three principal lines of land, specific identities 
