Philosophical Transactions. 229 



longing, on a comparison of their animals, to very different genera. 

 2. Of species belonging to the same natural genus inhabiting essen- 

 tially different situations. 



I. The shells of the genera Lottia and Patella are so extremely 

 alike, that Mr. Gray has not been able to find any character by 

 which they can be distinguished with any degree of certainty, yet 

 their animals are extremely dissimilar. The internal structure of 

 the shells likewise agrees. " Yet the animal of the 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, Monodontce, and Turbines, from which 

 it differs so remarkably in the simple conical form of its shell. 

 Similar instances of difficulty in forming distinctions occur in Pupa 

 and Vertigo, Vitrina and Nomina, Rissoa and Truncatella, 

 Siphonaria and Aucylus, Littorina and Assirninia, among the 

 univalves. The Mytilus ■polymorpkus, a fresh water species, does 

 not differ, as far as the shell is concerned, from any of the rnytili ; 

 but the animal is quite distinct. In the rnytili the lobes of the mantle 

 are free throughout nearly their whole circumference, while in the 

 M. polymorpkus they are united through nearly their whole extent, 

 leaving only three small apertures, one for the passage of the foot 

 and beard, and the other two for the reception and rejection of the 

 water. It must, therefore, form a new genus, which has been termed 

 Dreissena by Van Beneden. The Iridium and Anodonta- possess 

 similar shells, but quite different animals; so also Cytherea, Arterites, 

 Cyclas, and Pisidium. 



In many of these univalves it is impossible to distinguish the genera 

 without attending to the opercula, as in Pahuliua- and some specks 

 of Littorina, Phasiauella, and Neritinw as distinguished from Ne- 

 ritce. The genera of Bullia and Tcrebra are distinguished by the lip 

 in the former being large and expanded, and in the latter small and 

 compressed, occasioned by the different shapes of the feet ; the former 

 has also large and eyeless tentacles ; the Tercbne have small and 

 short tentacles, bearing the eyes near the lips. A similar difference 

 exists between Rostellaria and Aporrhais ( Strombus pes pehcam). 



II. The difficulties in this division the author considers under four 

 heads. 1. Where species of the same genus are found in more than 

 one kind of situation, as on land, in fresh and in salt water. 

 2. Where one or more species of a genus, most of whose species in- 

 habit fresh water, are found in salt or brackish water. 3. Where, 

 on the contrary, one or more species of a genus, whose species gener- 

 ally inhabit the sea, are found in fresh water ; and, 4. Where the 

 same species is found both in salt and fresh water. 



1. Auricula scarabus and A minima occur in damp places on 

 the surface of the earth ; A JudtB sandy places overflowed by the 

 sea- Amyosotis, A coniformis, A nxten»,.$c. (Conovulus), in 

 the 'sea; A Dombeyi, A facialis ( Clidiua), in fresh water. 



2. Lymnaceae commonly occur in fresh water; but L Baltlitru is 



