300 JOURNAL OF CONCHOLOGY, VOL. I4, NO. ID, APRIL, IQIS. 



The outstanding features here are the high development of the 

 endocone, and to some extent of the ectocone, of the third lateral in the 

 Banstead specimens with which is associated the occasional presence 

 of a small endocone on the first marginal. This last occurred, and 

 then only as a trace on one side of the radula, in only one of the 

 71 specimens from elsewhere. This is the more significant as these 

 included much larger individuals than the Banstead series and, as 

 was pointed out in my more detailed study of these specimens {supra 

 p. 218), this differentiation of the first marginal tooth increases as the 

 shell grows larger. This growth change is well shown in the figures 



4^6 

 z 3 



for the third lateral tooth from Tremadoc, Bicknor and Marple, but it 

 is less advanced than in the smaller Banstead specimens and prac- 

 tically never extends to the first marginal tooth. There also seems to 

 be a relative failure in development of the ectocone of the third 

 lateral tooth in the Marple specimens. 



With the alternative notation which was previously used {supra 

 p. 235), we get the following figures for the percentage occurrence of 

 each type of tooth in the 7 and 8 mm. series for the third lateral 

 tooth : 



Table V. 



These figures bring out equally well the greater differentiation of 

 the Banstead specimens. 



Two other features of the cutting points require notice, (i) The 

 Tremadoc specimens alone showed, in five individuals, the curious 

 condition known as " truncation of the mesocone " of the central 

 tooth, which seems to be actually a failure of development of the 

 highly refractile cutting point. In four this involved the mesocone 

 only (figure 5), the lateral cusps being fairly normal (figure 4) ; in a 

 fifth example (figure 6) the lateral cutting points were also absent. 

 (2) Five of the Bicknor specimens had a form of elementary ectocone 

 to the third lateral tooth which I have not seen elsewhere. The 



