168 J. E. S. MOORE. 



these teeth are highly suggestive of those of a number of well- 

 known forms. In gross detail the radular dentition is very 

 similar to that occurring in various forms of Anchy lotus 

 figured by Troschel. The predominant denticle on the 

 admedian tooth is well developed in zonatus, as indeed it is in 

 a very large number of dissimilar forms ; and an exactly 

 analogous and widely prevalent feature is presented in the 

 difference of size and character between the denticles on the 

 heads of the outer and inner lateral teeth. Perhaps, however, 

 the most notable feature which the radula presents is the 

 peculiar structure of the median tooth. The outer surface of 

 this tooth is concave, like the median tooth in Anchy lot us, 

 Thiara, Melania brevis (Dorb.),and Melanopsis. But it 

 differs from all these forms in having no predominant median 

 denticle, there being instead two lateral predominant denticles 

 and a median concavity. The only forms which appear to 

 possess this peculiarity of the median tooth are the different 

 species of the genus Sigaretus; and although in other respects 

 the radula differs widely from that of either a S igaretus or a 

 Natica, 1 shall show in the concluding part of the paper 

 that this comparison is not nearly so far-fetched as it might 

 appear, since it can be clearly demonstrated that what we may 

 call the melanio-planaxoid form of radula which S. zonata 

 possesses is that of an extremely ancient type, and in all 

 probability, with the exception of the Rhipidoglossa, is ante- 

 cedent to that of all the other Prosobianchiates with which 

 we are acquainted. 



In S. zonata the' oesophagus leads into a peculiar stomach, 

 represented in PI. 18, fig. 2, which contains at its anterior end 

 a body which must be regarded as representing a crystalline 

 style, although it is so small and so little elongated that at 

 first sight it does not seem to have the characters which this 

 structure usually presents. 



The intestine takes the course represented in PI. 18, fig. 6, 

 and communicates with a dilated rectum, opening in the usual 

 way just within the border of the mantle. The heart has the 

 regular tsenioglossate characters. The description of this 



