196 SHELLS AND SHELJL-FISH. PART I. 



somewhat uncertain. There are several fossil genera, 

 as Enomphalus Sow., OrUs, and Planaria* of Lea, of 

 a discoid shape, whose animals are quite unknown, and 

 will ever be so, if no recent species are discovered. 

 Their shells, indeed, are intermediate between those of 

 the Ampullarince and the Limnacince ; but then this dis- 

 coid shape is found in so many different families, that 

 their location here would be entirely conjectural. On 

 the other hand, we have our choice of Valvata and 

 Thallicera^: the first appears to us more like one of the 

 genera of the Ampullarina; while the extraordinary 

 animal of the latter, and the singularity of the shell, 

 leads us to view it, with M. Deshayes, more as the 

 representative of a sub-family than of a genus. May 

 this, again, be the natural station of the semi-aquatic 

 genera Melampus, Scardbus, &c., whose animals have 

 certainly a strong resemblance to those of the Limnacints ? 

 Into these theoretical questions we dare not enter ; 

 difficulties are opposed to the adoption of each of these 

 theories ; and we shall, therefore, choose that which 

 appears to us, upon the whole, least liable to objection, 

 namely, the supposition that Thallicera stands inter- 

 mediate between the Ampullarince and the Limnacince. 

 The animal of Thallicera, like the Limnacince, is her- 

 maphrodite : the head is large, flat, cleft in two lobes, 

 which bear the two sessile eyes ; but these are without 

 any appearance of tentacula ; the oper- 

 culum is horny; and the animal is ma- 

 rine. Such is the substance of the in- 

 teresting facts made known by M. Quoy, 

 who found the T. Avellana (fig. 34.) 

 in abundance on the coasts of New 

 Zealand. That it is thus allied both 

 to the operculated marine Pectinibranchia by its shell 



* This name cannot be retained, having been long ago applied to a well- 

 known genus among the Parenchymata of Cuvier, and of this work 



t M. Quoy, among his other brilliant discoveries in malacology, has the 

 honour of making known the animal of Ampullacera. I trust he will 

 excuse my proposing Thallicera, as a name not liable, like the above, to be 

 confounded with Ampullaria. 



