CYLTNDKELLA. 305 



first set apart by the Rev. Lansdowne Guilding, in 

 1828, under the generic name, Brachypus, derived from 

 the very short pedal disk when compared with the 

 length of the shell ; but as this name had been pre- 

 viously applied to other animals, he afterwards substi- 

 tuted for it the name Siphonostoma. The same year Dr. 

 Pfeiffer applied to it the name Cylindrella. These 

 names are both highly descriptive of the shell ; but, as 

 Guilding's second name had also been previously in use, 

 and Dr. Pfeiffer' s name has generally obtained, we also 

 adopt it. The characters of this group appear to be 

 sufficiently prominent and natural, both in respect to the 

 shell, the animal, and its circumscribed range. The 

 shell has been hitherto ranked with Pupa, Cyclostoma, 

 and Clausilia. It resembles the latter in its form, but 

 lacks the clausilium, and has, rather, the aperture of 

 Cyclostoma ; there is, however, no operculum. The 

 animal differs from that of Cyclostoma in the number of 

 tentacles and position of the eyespots ; it is, however, 

 closely allied to the animals of Clausilia and Pupa. 



The apicial nucleus of the shell is a small globule ; 

 this is succeeded by a large number of closely revolving 

 whorls of still smaller diameter, which scarcely augment 

 in length ; and then there is a rapid dilatation to the full 

 size of the shell. At this part, either by fracture, or 

 more probably by absorption, the slender tip is thrown 

 off, so that we have only the truncated lower portion left. 

 In many species the last whorl stretches out into a long 

 neck ; in others, however, this uncoiling is very slight, 



VOL. ii. 39 



