Extmct Selachian Asteracanthus. 341 



founding of a new species. Mr. Leeds's collection, however, 

 comprises so large a series of dorsal spines from the Oxford 

 Clay, and these exhibit so many variations in ornament — 

 graduating from the type shown in figs. 5 and 6 to that of 

 the most characteristic A, ornatissimus — that it seems quite 

 impossible to discover any line of specific separation in the 

 series. Indeed, the more the dorsal fin-spines of sharks are 

 studied, the more impossible does it appear to employ varia- 

 tions in their surface-ornament for specific diagnosis ; and it 

 yet remains — at least in Asteracanthus — to determine what 

 are the precise characters in the dentition to be counted as of 

 real value. The teeth of series 3 and 4 in group no. 4 are 

 relatively narrower and exhibit a more prominent coronal 

 eminence than the corresponding teeth associated with the 

 typical spine of A. ornatissimus in group no. 3. Fig. 9 

 represents a tooth, probably of series 3, of the latter, and 

 fig. 10 two abnormal transversely-divided teeth, evidently of 

 series 4 of the same fossil, and these agree more closely with 

 the typical Kimmeridgian teeth from IShotover than those of 

 the fish with finely ornamented fin-spines from Fletton. 

 The general fades of the dentition, however, is identical in 

 the two forms, and it thus seems most reasonable at present 

 to describe the new one merely as a hitherto unrecognized 

 variety under the name of Asteracanthus ornatissimus^ var. 

 flettonensis . 



Conclusion. — From a study of the fossils just described it 

 may be inferred, with much probability of correctness, that 

 all the " species " of teeth named Strophodus are referable to 

 the Selachian Asteracanthus. The spines and the teeth are 

 often found together upon certain horizons ; and the few cases 

 in which they have not been thus discovered are worthless 

 for consideration, being mere negative evidence. Astera- 

 canthus verrucosus^ for example, is common in the Purbeck 

 Beds of Swanage, while not one tooth of ^^ Strophodus " 

 appears yet to have been discovered there ; but spines of 

 Hyhodus are still more abundant in those beds, and it is 

 scarcely minimizing to state that in all the public collections 

 of Britain not more than a dozen teeth of this shark are to be 

 seen from the same horizon. 



The zoological result of this brief study is also interesting, 

 demonstrating a still more close relationship between Astera- 

 canthus and Hyhodus and Acrodus than has hitherto been 

 suspected. No vertebra have been discovered with the 

 remains of the Oxfordian genus, and it thus probably pos- 

 sessed a persistent notochord, like Hyhodus and Acrodus. 

 The dentition is fundamentally the same, only distinguished 



Ann. (& Mag. N. Hist. Ser. 6. Vol. ii. 24 



