20 AMERICAN JOURNAL 



Their exterior form, coloration, system of ornamentation, the 

 position of their branchige, contribute to ally them with the Doris. 

 They are scarcely distinguishable externally by thesmallness of 

 the head and mouth, and by a certain difference in the position 

 of the latter. However, they show no sign of lingual dentition, 

 jaw, nor spinose collar, and their mouth, designed to act merely 

 by suction, possesses a retractible proboscis. It is well known 

 that such is not the case with the real Doris. In following to 

 the letter the system upheld by Mr. Morch, it would be neces- 

 sary, for this single difference in the lingual dentition, to put very 

 far apart and in different orders these Mollusks, which are scarce- 

 ly generically distinct in all other particulars. Such a result, 

 we tliiiik, cannot be appropriately considered as the evidence of 

 a natural system. 



We may add that the lingual dentition varies considerably in 

 Gasteropodous Mollusks, not only from genus to genus, but still 

 more from species to species. The radula of the Cyprcea, of the 

 section Trivia, (C. coccinella, for example,) resembles much more 

 that of the Erato and even Pedicularia than that of the Cyprcea 

 of the other sections, which, however, are near relatives.* Still 

 more, the lingual dentition does not always present characters 

 constantly identical in the same specific type. Thus Mr. 

 Troschel f has given the figure of two median plates, belonging 

 to two individuals of the same species, the Natica adspersa, 

 Menke, and presenting between them marked differences. 

 The same takes place in the lingual dentition or jaw. 



Mr. Binneyl has recently figured nine jaws, all from adult 

 specimens of an American species, Helix Tryoni., Newcomb, and 

 in which the number, the dimensions and the relative position of 

 the ribs of the anterior surface present such dissimilarities, that 

 each of them seems to belong to a distinct species. 



Finally, while genera, conchologically and zoologically neigh- 

 bors, sometimes possess a great variety in their lingual dentition, 

 on the other hand, generic forms, distant from each other, 

 present between themselves, singular analogies in the construc- 

 tion of their radulce. Thus we see a great similarity between 

 the lingual dentition of the Vermetus, Natica and Crucibidum,^ 

 although thi-se genera are very distant from each other. 



The median plate of the radula of the Velutina might almost 

 be taken for that of the Pedicularia. These two genera could 

 scarcely, however, be brought together. 



What conclusion can we draw from all this, if not that the 

 characters drawn from the lingual or buccal dentition of the 



* Troschel, Das Gebiss der Schnecken, I. pi., XVI, XVII, and XVIII. 



t Troschel, 1. c. I, pi. XIV, p. 13 and 13a. 



X American Journal of Uonch., Vol. I, pi. VI. 



I Troschel, 1. c. I, pi. XIII, fig. 1, 3, 4, 5, and pi. XIV, p. 13. 



