72 H. L. Hawkins — Jaw-structures in Conulus. 



some little distance within the peristome the chalk filling is soft and 

 crumbling, but otherwise it is very hard. 



After slicing off one side of the specimen I was able to develop the 

 peristomial region in such a way that it could be viewed from the 

 side. The appearance of the teeth after this operation is shown in 

 PL III, Fig. 1. I subsequently extracted two of the teeth for 

 detailed examination, and the following description is founded 

 on these two examples only. The others seem identical in 

 character. 



The teeth are composed of extremely hard calcite with a porcellanous 

 lustre. The more complete of the two specimens (PL III, Figs. 2-5) 

 measures 6 - 7mm. from the (accidentally) blunted tip to the inward 

 end of the internal carina. The other is 7'0 mm. long, and has the 

 point intact, but the rest of the tooth is not very well preserved. 

 Except towards the tip, where for a distance of 1 mm. they taper to 

 a point, both teeth have a uniform width of 1*4 mm. 



The curvature of the whole tooth is very slight, as shown in Pig. 2. 

 The convex surface is marked by a fairly prominent ridge (witli 

 angular margins) down the middle line, and two less conspicuous 

 ridges down the edges. On the concave surface, which is slightly 

 hollowed, there is an extremely prominent carina, which rises abruptly 

 at a distance of 1 mm. from the point of the tooth, and in both specimens 

 persists upwards beyond the blade. 



It will be noticed that the above description agrees fairly closely 

 with that given by Forbes in 1850. The teeth are strikingly similar 

 to those of Biscoidea cylindrica which I described a short time ago 

 (Geol. Mag., 1909, p. 149, PL VI, Fig. 4). The length of these new 

 specimens is greater than that in the case of Biscoidea, but owing to 

 the fact that all are broken, this is a point of no importance. The 

 width of the Biscoidea tooth is only -9 mm., as against the l - 4 mm. in 

 the Conulus examples. The extreme sharpness of one of the Conulus 

 teeth shows a contrast to that of Biscoidea, but the bluntness in the 

 latter case (certainly not due to fracture) may have been the result of 

 wear. The most important difference between the two genera, in the 

 matter of their teeth, is the almost straight shape of the tooth in 

 Conulus. I have copied the figure of the side view of the Biscoidea 

 tooth (reduced to the same scale as the others) to render this contrast 

 more obvious. See PL III, Fig. 6. 



In spite of the most careful search and very gradual development of 

 the specimen, I found no trace whatever of jaws or any other 

 structures beside the teeth. Although it seems impossible that teeth 

 of such dimensions could exist, and especially be functional, in the 

 absence of jaws for their support, yet it is almost less conceivable that 

 they could have remained within the test while the jaws slipped 

 through the peristome after the animal's death. An explanation of 

 this apparent anomaly might perhaps be found in a suggestion that 

 the jaws had advanced so far in degeneration that they were no longer 

 calcareous, but I know of nothing in recorded jaw-structures that 

 would support such a view. 



The presence of teeth in Conulus subrotundus is established beyond 

 doubt by the discovery of this specimen, but the question still remains 



