302 MR. GRAY ON TESTACEOUS MOLLUSCA. 



1 . Of Shells apparently similar, hut belonging, on a comparison of their Animals, to 



very different Genera. 



In a note on my former paper on the structure of shells *, I pointed out the per- 

 plexity 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 re- 

 gard to several other genera. The two genera above referred to are probably, how- 

 ever, the most remarkable example of this complete resemblance, on account of the 

 extreme dissimilarity of their animals, which are referrible to two very different 

 orders of Mollusca, while the shells are so perfectly 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 to- 

 wards 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 structure 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 inhabitants 

 of the Trochi, Monodontoe, and Turbines, from which it differs so remarkably in the 

 simple conical form of its shell. This difference in the respiratory organs of animals 

 inhabiting shells so strikingly similar is the more anomalous, inasmuch as those 

 organs commonly exercise great influence on the general form of shells ; a circum- 

 stance 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. 



A similar difficulty exists with regard to Siphonaria and Ancylus, genera belonging 

 to two different families, one inhabiting the sea-shores, while the other lives in rivers 

 and brooks. The only distinction between the shells of these two genera consists in 

 the Ancyli being generally of a thinner substance than the Siphonarice ; but this is 

 by no means an adequate character, some species of Siphonaria {S. Tristensis, for 

 example,) being quite as thin in texture as any Ancylus. Both have the muscular 

 impression interrupted by the canal through which the air passes to the respiratory 

 organs ; yet the animal of Ancylus has long tentacles, and eyes placed as in the 

 Lymncece, to which it is closely allied, while Siphonaria has no distinct tentacles, and 



* Philosophical Transactions, 1834, p. 800. 



