Mr Gray on Testaceous MoUiLSca, 81 



1. On Shells apparently similar ^ but 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 perplexity in which the extreme similarity of 

 the shells belonging to the genera Patella and Lottia must in- 

 volve the geologist and the conchologist, intending at some fu- 

 ture time to pursue the subject further, and to shew that simi- 

 lar 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 dissimilarity of their animals, which are referable 

 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 ge 

 nera 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 ii^ 

 both the orders to which they belong, offering no variation in 

 this respect from the usual structure of the class. The agree- 

 ment 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 Trochif Motiodonta^, 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 circumstance readily accounted fOT 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, Viirina and Nanina, Rissoa and Trunca- 

 teUa, as affording numerous and perplexing instances of the 



• Philosophical Transactions, 183 i, p. 800. 

 VOL. XX. NO. XXXIX.— JANUARY, 1836. F 



