H. L. Hawkins — Jaw-structures in Conuhis. 73 



as to whether similar structures existed in C. albogalerus. I have 

 indicated above the strong negative evidence afforded by their absence 

 in fifty specimens, in one of which the buccal plates remained intact. 

 But such evidence is, of course, unsatisfactory. I can find no record 

 of buccal plates having been detected in C. subrotundus, nor have 

 I found traces of them among several hundred specimens, many very 

 perfectly preserved, that I have examined. The perignathic girdle 

 in this species is similar to that in C. albogalerus, but I have not 

 found the small adradial process of the latter represented in the 

 specimens dissected. 



Duncan based part of his argument against the existence of jaws 

 and teeth in C. albogalerus on the assumption that the buccal plates 

 narrowed the mouth aperture to such an extent that there was no 

 room for jaws to exercise their functions. The preparation of the 

 peristome in the specimen above referred to, from the upper part of 

 the zone of M. coranguinum (PL III, Pig. 7), shows the buccal plates 

 from within, a view which, I believe, has not hitherto been described. 

 Each plate has a deep pit in a groove towards the ambulacral margin. 

 (Externally there are two tubercles in sunken areolae, like those of 

 the test.) The plates are roughly triangular, and each pair is in 

 contact along almost the whole interradial margin. The position of 

 the plates with regard to the grooves in the perignathic girdle suggests 

 that they could slide up these, and so enlarge the diameter of the 

 mouth opening. If this were the case, then perhaps a muscle was 

 attached in the pit on the plate, and passed to the small adradial 

 process of the girdle. Such an arrangement might conceivably be 

 a modification from the characters and muscle attachments of the 

 pyramids of a lantern, and then each buccal plate would be a degenerate 

 demi-pyramid. The presence of tubercles on the outer surface is the 

 only feature seriously antagonistic to this view. As buccal plates of 

 this kind are not known in any other genus or species, living or extinct, 

 it seems probable that the development of these tubercles is secondary, 

 consequent on the external position of the plates, and that the buccal 

 plates really are curiously modified remains of the jaws. If this were 

 to be demonstrated by future discoveries, it would be difficult to 

 understand how teeth could find their place in such pyramids, 

 although the retractability of the plates would remove the difficulty 

 of the small size of the oral aperture. 



If buccal plates were to be found in C. subrotundus, there would be 

 no reason why teeth should not occur in C. albogalerus, but in the 

 absence of these structures in the earlier species, the following 

 summary of the suggestions made here may show some approximation 

 to the actual conditions. 



Summary. Conulus subrotundus had teeth, of a type differing but 

 little from those of Biscoidea cylindrica, but the remaining parts of 

 the jaw apparatus were either uncalcified or too delicate for 

 preservation. Conulus albogalerus, developing along a somewhat 

 different line, lost its teeth, but retained the pyramids of the jaws, 

 modified to form buccal plates. 



In conclusion, I wish to express my thanks to Dr. P. A . Bather, P. U.S., 

 for his help and advice, and to Mr. T. H. Withers for the gift of several 



