LINGUAL DENTITION OF SOME WEST-INDIAN GASTEROPODA. 193 



remarkable for the great length of the imbricating pellucid cusps. The laterals seem also 

 to have a similar cusp ; but it appears to be more often absent than present : as, however, 

 their extreme fragility renders them very likely to be broken off, it is in all probability 

 due to this cause that they are so often found wanting. 



Macroceramm signal us (fig. 7) resembles Helix more than any of the Jlidimi here 



described in its dentition. The medians are elongate, with a single obtuse rounded 

 cusp; the laterals have two round subequal cusps, of which the inner one is situate on 



the side of the tooth. 



Pupa striatelhi, a species inhabiting Porto Rico and other West-Indian islands, has a 

 dentition somewhat resembling that of Vidimus. The teeth are obtusely tricuspid, the 

 medians not any smaller than the laterals. Succwea o p p rox i m au t has teeth possessing 

 characters of Bulimus, but differing in details, as will be seen by the specimens. In 

 Veronicella (Vaginulus) Slomiei the medians are small, and the laterals as well as the 

 medians are tricuspid. This dentition does not resemble that of Oitcidium, with which 



the genus has hitherto been associated. 



In Physa rivalis the teeth are slender, and bear no great resemblance to those of any 

 of the terrestrial forms. In Pkniorbis terversunus the median teeth are largest, and 

 have three acute points, of which the middle one is the smallest. The laterals have 

 three acute points, of which the outer one is largest. The edges of the teeth are not 

 renexed. Amongst the marine mollusca which have teeth somewhat of this shape are 

 Voluta and Mar •ginella ; but these have very much fewer teeth, with usually more 



numerous denticles. 



Marginella cceridescens (fig. 3) has a small subquadrate band of acute pellucid teeth, 

 the medians of which are shortest ; the laterals are alternately long and short, set on a 

 delicate homogeneous basement membrane. 



n 



Streptaxi* defonnis (fig. 6) has aculeate teeth, resembling those on the lateral portion 

 of the dental membrane of Zonites cellarius. The young shell is like a Zonites ; so that 

 we may readily infer the close affinity of these genera. 



The dental apparatus of Cyl'mdrella trinitaria (fig. 12) is a singular modification of the 

 inoperculate type. The band is very long, resembling in this respect that of LUtorina. 

 The individual teeth have a certain likeness to those of Helix, but they also recall to our 

 minds, in their mode of arrangement, the Operculata. 



The Inoperculata seem hardly to furnish us with much evidence as to their alliances 

 with truly marine genera ; but the operculate land-shells, on the contrary, exhibit un- 

 mistakeable affinities with several groups. As we have previously remarked, Helicina 

 shows relations with Nerita: a comparison of them with N. microstoma (fig. 9) will 

 prove the fact. A study of the Ungual teeth has further made obvious what we 

 might have expected from other characters, that the Cyclostomatidae must be sepa- 

 rated altogether from the Cyclophoridse. The latter have teeth resembling those of 

 LUtorina. As examples we may cite Cyclotus translucid ns (fig. 11) and C. rugatiis. 

 This dentition is represented by the formula 3.1.3. The animals of the Cyclosto- 

 matidas, on the other hand, have a foot divided into two longitudinal portions by a 

 ^roove As examples of their dentition (00 . 2 . 1 . 2 . 00) figures are given of Cistula pup*. 

 b 2d2 



