DISSIMILAR ANIMALS IN SIMILAR SHELLS. 403 



" Of Shells apparently similar, but belonging, on a comparison 

 of their Animals, to very different Genera." By JOHN 

 EDWARD GRAY, Esq., F.R.S., &c. (Reprinted from the 

 Philosophical Transactions, part ii. \ 835.) 

 " In a note on my former paper on the structure of 

 shells,* I pointed out the perplexity in which the extreme 

 similarity of the shells belonging to the genera Patella and 

 Lottia must involve the geologist and the conchologist, 

 intending at some future time to pursue the subject further, 

 and to show that similar difficulties existed in regard to 

 several other genera. The two genera above referred to are 

 probably, however, the most remarkable example of this 

 complete resemblance, on account of the extreme dissimi- 

 larity of their animals, which are referable to two very 

 different orders of Mollusca, while the shells are so per- 

 fectly alike, that after a long-continued study of numerous 

 species of each genus, I cannot find any character by which 

 they can be distinguished with any degree of certainty. 

 Both genera present a striking discrepancy from all other 

 univalve shells, in having the apex of the shell turned 

 towards the head of the animal, the genera to which they 

 are immediately related in both the orders to which they 

 belong offering no variation in this respect from the usual 

 structure of the class. The agreement in the internal struc- 

 ture of their shells is equally complete ; yet the animal of 

 Patella has the branchiae in the form of a series of small 

 plates disposed in a circle round the inner edge of the 

 mantle, while that of Lottia has a triangular pectinated gill 

 seated in a proper cavity formed over the back of the neck 

 within the mantle, agreeing in this respect with the inha- 

 bitants of the Trochi, Monodontae, and Turbines, from 

 which it differs so remarkably in the simple conical form 

 of its shell. This difference in the respiratory organs of ani- 

 mals inhabiting shells so strikingly similar is the more ano- 

 malous, inasmuch as those organs commonly exercise great 

 influence on the general form of shells ; a circumstance readily 

 accounted for when we reflect that a principal object of the 

 shell is to afford protection to those delicate and highly 

 important parts. 



" To the practical conchologist it will be sufficient to 

 mention Pupa and Vertigo, Vitrina and Nanina, Rissoa and 

 Truncatella, as affording numerous and perplexing instances 

 of the difficulty of distinguishing between genera of shells, 

 inhabited by very different animals. 



* Philosophical Trans. 1834, p. 800. 



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