128 



HE. A. S. WOOE-WAED ON THE DENTITION 



eminence with radiating furrows ; while in the latter (fig. 12) the 

 same part of the grinding-surface is represented by about three very 

 short, comparatively broad ridges, and the anterior granulated 

 portion is marked by delicate branching grooves. 



VIII. A concluding note may also be added upon a large median 

 tooth of P. decurrens (fig. 13), which shows the extent to which abra- 

 sion sometimes proceeded before the final shedding. The scar caused 

 by the opposing row of diminutive median teeth has assumed extra- 

 ordinary proportions, and there are also three well-defined lateral 

 marks of wearing. Two of the latter are observed on the left, and 

 a larger one, connected with the median, is situated somewhat 

 posteriorly to the right. 



Conclusions. 



On taking a general glance at the fossils just described, and 

 considering their peculiarities as mutually illustrative, it seems 

 possible at last to determine the precise character of the dentition of 

 Ptychoclus, at least so far as one species is concerned. The specimens 

 numbered II. and V. (figs. 1 and 4) show distinctly the plan of ar- 

 rangement of the two jaws, and the other small fossil No. III. (fig. 2) 

 will obviously determine the number of parallel rows constituting the 

 complete armature. Unfortunately, however, it is somewhat difficult 

 to decide whether the latter specimen is truly referable to the jaw to 

 which I supposed it to belong when my previously published diagram 

 was sketched, or whether it rather appertains to the new type of 

 dental arrangement revealed by Mr. Willett's fossil. It is un- 

 doubtedly an example of P. decurrens, and the rows marked 1 and 

 2 exhibit precisely the same relative proportions as those similarly 

 denoted in specimen No. IV. (fig. 3) ; while there is no transverse 

 elongation such as is evident in the most fragmentary jaw of No. V. 

 (fig. 4). The larger tooth, on the other hand, does not agree very 

 well either with the original of fig. 6 or that of fig. 8 ; but, on 

 the whole, it seems most likely that the former is its homologue. 

 Such being the case, there would be originally seven lateral rows on 

 either side of the small median series, and the opposing jaw would 

 almost certainly comprise an equal number. If, however, the large 

 tooth in fig. 2 is one of a median row, there will only have been 

 six lateral series in each. 



Having determined so much, it requires but little study to demon- 

 strate that the dentition is that of a true Rav, and does not bear the 

 slightest resemblance to that of the Cestraciont Sharks. Though, 

 at first sight, the specimen No. II. (fig. 1) might suggest its apper- 

 taining to a single ramus of* the jaw — with the largest series in the 

 middle, much as in the living Cestracion — yet the absence of all 

 obliquity in the rows, and their completely symmetrical character, are 

 features that at once forbid this interpretation ; and Mr. Willett's 

 specimen (No. V.) furnishes decided proof that such an idea is quite 

 inadmissible. The two mandibular rami were thus placed almost 

 in the same straight line, as is the case in so many of the living' Kays, 



