198 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



The central teeth, also, are rather smaller than those immediately 

 flanking them. The teeth of the upper jaw are precisely similar 

 to those in the lower. In neither do we find any material increase 

 of obliquity in the cusps as they recede from the centre. The 

 cartilaginous alee of the mouth are distinctly traceable ; they in- 

 crease in width rapidly from the symphysis of the lower jaw, and 

 attain their maximum expanse at the angle of the mouth. Behind 

 these are some traces of the hyoidal arch. It is probable from the 

 appearance of the matrix which envelopes it, that with a little care- 

 ful cleaning a considerable portion of the head might be disclosed. 

 In its present condition, the only part of the cranial cartilages to 

 be distinguished is a section of the prosencephalic cavity. 



The geological inferences afforded by this specimen are briefly 

 told. The species is new. The genus is undoubtedly Hybodus., 

 This genus attains its maximum expansion in the Oolitic series., 

 but it ranges from the Muschelkalk to the Chalk inclusive. The 

 only evidence of its occurrence in the latter formation is a frag- 

 ment of an Ichthyodorulite in the Mantell collection. The teeth 

 have not yet been found in any strata more recent than the 

 Wealden. As far therefore as the evidence goes, it leads to the 

 supposition that a bed containing teeth of the genus Hybodus is 

 most likely to be of an age anterior to the cretaceous system. In a 

 zoological point of view, this specimen is of more importance, in- 

 asmuch as it fully corroborates the views advanced by Agassiz, 

 " that most probably the Hybodonts differed little from the recent 

 sharks in general aspect." It also authenticates the numerous 

 species established by that distinguished naturalist from the cha- 

 racters of isolated teeth. We find in many of the recent sharks, 

 in Carcharias for example, the discrepancy between the teeth of 

 the upper and lower jaw so great, that it would be considered 

 quite warrantable to describe them, if found detached, as different 

 species. It was from a just appreciation of these difficulties that 

 Agassiz has always professed his names and characters of the 

 placoid teeth to be descriptive of specimens, and to be considered 

 provisional as regards specific arrangement, until evidence should 

 be found authorising or annulling the continuance of the titles 

 as applicable to species. Mr. Ibbetson's specimen shows that in 

 the genus Hybodus there was no difference between the teeth of 

 the upper and lower jaw, and less variation, according to position, 

 than in the recent sharks ; consequently the descriptive characters 

 given in the Poissons Fossiles will hold good as specific distinctions. 

 The Hybodonts, then, of the secondary strata differed only from 

 the sharks of the recent period in those modifications which adapted 

 them to the circumstances under which they existed. The form 

 of the mouth was nearly similar ; from this we may argue a simi- 

 larity of shape and an analogous arrangement of the fins to enable 

 them to seize their prey. If they subsisted upon fish, which is 

 most probable from the form of the teeth, we find in the denser 

 structure and hard enamel coating of these organs, provisions to 

 enable them to grapple with the Ganoid fishes of that period ; 

 while in the powerful fin bones with which they were armed, we 



