Remarks on Mollusca and Shells. 75 



California (L. undulata) has the thin, milk white, concentric- 

 ally undulated valves so similar to those which characterize 

 a shell from the coast of Carolina (L. canaliculata), that no 

 one observing them separately would hesitate to pronounce 

 them the same ; but place the two side by side, and it will 

 be seen that in one the beaks are near the posterior, and in 

 the other near the anterior end of the shell. Equally strik- 

 ing resemblances and differences will be found when we com- 

 pare Mactra nasuta and M. braziliana, Lutraria ventricosa, 

 and L. carinata, the former of which are found in the Gulf 

 of California, and their analogues in the Gulf of Mexico. 

 So, too, we find on the catalogues Cytherea chione and 

 Natica maroccana, Mediterranean shells, set down as found 

 also in the Gulf of California ; but a direct comparison shews 

 them to be quite different in form and coloration, and well 

 entitled to the distinctive appellations of Cytherea biradiata 

 and Natica Chemnitzii. Triton nodosum, of the West Indies, 

 has also been regarded as identical with a Sandwich Island 

 species (T. elongatum). We need not multiply examples of 

 this kind. But if such confusion has arisen among strongly- 

 marked species, how much more liable is it to occur where 

 specific differences are slight 1 In many genera, as in Physa 

 and Succinea, the form, surface, and colouring, are so uniform 

 throughout, that undoubted species are distinguished by only 

 the slightest differences. Indeed, there are even some 

 genera, like Helix and Nanina, Patella and Lotiia, which 

 cannot be distinguished but by an examination of the animal. 

 When, therefore, we have before us shells from widely 

 diverse regions, apparently identical, they should be sub- 

 jected to the most careful scrutiny for structural differences. 

 If no obvious ones are detected, we may not consider the 

 question as settled, unless the animals have been compared ; 

 and we may go even further, and require that their internal 

 structure, as well as external features, should be ex- 

 amined. The number of instances where this apparent 

 ubiquity exists is fast diminishing, as in the cases already 

 mentioned, in those of Cyprea exanthema, cervina, and cer- 

 vinetta, &c. A large proportion of the shells inhabiting the 

 eastern and western shores of the Atlantic have been re- 



