ON MOLLUSCA OF THE WEST COAST OF NORTH AMERICA. 363 



Now even Prof. Adams allowed that one shell was common, viz. Crepi- 

 dula unguiformis* ; and Dr. Gould himself inserts Venus circinata and 

 Crepidula aculeata in his Mexican War Lists. We therefore naturally 

 argue, if one may be common, why not others also ? Because we cannot see 

 how they should find their way to other seas, is only an argument drawn 

 from our ignorance. Prof. Forbes, on glancing over the list of the Reigen 

 Collection, allowed that there might be species in common ; and in the 

 ' Quarterly Journal ' of the Geological Society will be found a paper by 

 Mr. Henniker, in which the author gives geological reasons for the pro- 

 bability of the intercommunication. As the level of the Atlantic is higher 

 than the Pacific, any such communication must have poured the treasures of 

 the Atlantic into the Pacific, and scarcely allowed of an exchange in the 

 other direction. Such is found to be the case ; no species fairly belonging 

 to the exclusive Pacific fauna being found in the West Indies. Is it possible 

 that some such intercommunication may have been correlative with the 

 glacial conditions of the European seas ? Some of the supposed Caribbean 

 shells in the Pacific appear to have migrated northwards ; the Cyprcea ex- 

 anthema being poor and small at Panama, where it is called C. cervinetta, 

 but large, fine and tolerably abundant at Mazatlan ; the Strigilla carnaria 

 also, not even noticed as an analogue by Prof. Adams, appears blanched but 

 not uncommon at Mazatlan, and having crossed the " Cape Codf " of the 

 western shores, assumes its normal condition on the Californian coast. The 

 ubiquitous Purpura patula, unknown at Panama, is extremely fine at the 

 Gulf. Other species, however, seem to be dying out ; as Lucina tigerrina 

 and Mactra fragilisi 



A. Species regarded as identical between the Pacific and Atlantic. 



Pacific. West Indies. 



1. Gastrochsena truncata ... sp. — BristolMus. 

 2. ovata sp. — BristolMus. 



3. Petricola cognata pholadiformis. 



4. Tellina simulans punicea. 



5. rufescens operculata. 



6. vicina bimaculata. 



7. Strigilla fucata carnaria. 



8. pisiformis, teste Phil, pisiformis. 



9. Mactra fragilis fragilis. 



10. Dione circinata (? + al- circinata. 



ternata. 



11. Lucina tigerrina tigerrina. 



12. Diplodonta semiaspera . . . semiaspera, teste 



Phil. 



13. Modiola Braziliensis Braziliensis. 



14. Lithophagus aristatus ... caudigerus. 

 15. cinnamomeus cinnamoraeus. 



16. Area labiata labiata. 



17. Isognomon flexuosum ... Chemnitzianum. 



18. Ostrea Virginica Virginica. 



19. Placunanomia foliacea ... foliacea. 



Pacific. West Indies. 



20. Orthalicus zebra undata. 



21. Hipponyx antiquatus mitrula. 



22. Panamensis subrufa. 



23. Crepidula hystrix l aru leat a 



echinus |aculeata. 



24. unguiformis Goreensis. 



25. Crucibulum Cumingii ... sp. 



26. Ovulum gibbosum, teste gibbosum. 



Cuming. 



27. Cyprsea cervinetta exanthema. 



28. Torinia variegata variegata. 



29. Leiostraca Pdistorta Pdistorta. 



30. Olivella zonalis sp. 



31. Marginella cserulea prunum. 



[not sapotilla]. 



3 2. Nitidella guttata cribraria. 



33. Purpura pansa patula. 



34. Anachis pygmaea costulata. 



35. Pisania ringens sp.[Pernambuco, 



Br. Mus. Per- 

 haps error.]. 



It will be seen that more than half the marine shells are bivalves. 



* It is generally said that this shell is only a variety of local types. Each local white shell 

 may take the form unguiformis ; but there remains a distinct type, known by the form of 

 the vertical whirls, which appears to be ubiquitous. It is not always recurved, and in its 

 natural state appears to be the Patella Goreensis of Gmel. — Vide Plate. 



f This Cape separates the two faunas in Massachusetts : Cochlodesma, Montacuta, Cumingia, 

 Corbula, Tornatella, Vermetus, Columbella, Cerithium, Pyrtda, Ranella, do not pass north- 

 wards ; nor Panopaia, Ghjcimeris, Terebratula, Puncturella, Trichotropis, Aporrhais, nor Admete 

 southwards. Of 197 marine species, 83 do not pass to the south, and 50 are not found on the 

 north: 70 are found in Europe. {Gould, Rep. Inv. Mass.) 



