Mr. J. Alder on the genus JefFreysia. 195 



different from this is the operculum of Jeffreysia. The nucleus 

 is central, or equally distant from both ends, and placed close to 

 the inner margin; and from it concentric lines of growth are 

 seen to emanate. On the side next the pillar there is a strong 

 rib, from which a process rises at right angles to the opercular 

 disc, projecting internally. It is thus noticed by Mr. Clark : 

 " It (the operculum) has marked strise of increment proceeding 

 from a minute apophysis, which is the nucleus." In what position 

 is not mentioned. That the apophysis or process, which is large 

 in proportion to the disc, is not the nucleus of the operculum, T 

 think any one may satisfy himself by a careful inspection. The 

 nucleus is the point on the side of the disc from which the con- 

 centric lines of growth oi*iginate. The lines of growth on the 

 apophysis inci'ease in an opposite direction. 



We now come to the soft jjarts of the animal, which, accord- 

 ing to Mr. Clark's views, can alone furnish generic characters. 

 The head of Jeffreysia is elongated into a kind of muzzle, which 

 is cleft in front and produced into two tentacular processes ; the 

 mouth has a pair of denticulated jaws, and a spinous tongue, 

 similar to what is seen in Rissoa and other phytophagous gaste- 

 ropods, to which tribe it belongs. The head of Chemnitzia is 

 very short, without muzzle or additional tentacular processes; 

 the mouth has no jaws, but is furnished with a long, retractile 

 proboscis, as in thg zoophagous gasteropods ; and there are no 

 spines on the tongue, or at least none have ever been detected. 

 The true tentacles in Jeffreysia are linear and a little flattened : 

 those of Chemnitzia are ear-shaped or longitudinally folded ; a 

 peculiarity confined to this group among the testaceous moUusca. 

 The eyes in the latter are sunk in the head at the inner angles of 

 the tentacles, appearing externally as black spots : the eyes in 

 Jeffreysia are largely developed, raised on slight bulgings, and 

 placed on the back a considerable distance behind the tentacles. 

 The foot in Chemnitzia is furnished with a conspicuous fold on 

 the upper anterior surface, generally bilobed, forming what 

 M. Loven calls the mentum. The use of this organ is not un- 

 derstood, but in some genera it shows a folliculated structure 

 internally. The mentum is absent in Jeffreysia. Mr. Clark, in 

 all his descriptions of the animals of Chemnitzia, has made the 

 mistake of taking this organ for the muzzle, and hence his com- 



phana, " the nucleus," he says, " being at the centre of the pillar edge." This 

 is not the case in my specimens. Again, in describing another Chemnitzia, 

 which he supposes to be my Odost. nitida (Brit. Moll. vol. ii. p. 282), Mr. 

 Clark says, the structure of the operculum is nltogether similar to that of 

 Jeffreysia diaphana. The other part of the description, however, seems to 

 contradict this, as it is stated that " the strise of increment radiate conspi^ 

 cuouslv to the outer margin." 



13* 



