258 A. Smith Woodivard — On the Genus Notidanus. 



at the base of the crown. Fig. 2 is a smaller but very similar tooth, 

 and the characters of both these specimens are such that they might 

 well be looked upon as closely allied to ancestral Notidanidge, even 

 if not actually upon the main line of descent of this family. 



Another interesting fact worthy of consideration is the distinct 

 evidence of wear already noted in the dentition of the N. eximins of 

 Schnaitheim (p. 210) ; it is impossible to conceive of this taking place 

 in Sharks that have the teeth so loosely attached to the jaw, and so 

 frequently replaced as are those of the living Notidanidaa, Lanmidas, 

 Carchariidas, etc., and I have never observed the peculiarity either 

 in the Eecent, Tertiary, or Cretaceous forms. The circumstance is 

 very suggestive, indeed, of the oldest Notidanidaa having the teeth 

 as firmly implanted as those of the Hybodonts, in which there were 

 two or more rows in function at a time, and in which also the longest 

 cones often show most evident traces of abrasion. The shape of the 

 root is quite in accordance with this supposition. It is proper to 

 add, however, that I have met with no very decided abrasion among 

 the large series of British Museum specimens of H. polyprion, which 

 seem most nearly to approach Notidanus, although this fact may be 

 partly accounted for by the considerable rolling to which many of 

 the teeth have been subjected before entombment in the Stonesfield 

 Slate ; the cones of the Carboniferous Cladodus are often much worn 

 down, and so likewise are those of the long-coned species of Hybodus 

 in the Muschelkalk. 



As to the precise significance of these various points of similarity 

 between the Hybodonts and the primitive Notidanidas, it would be 

 unsafe at present to express any definite opinion. Except in the 

 structure of the skull, which is very similar, 1 the latest members 

 of each family exhibit many important differences. The Hybodonts 

 have two (spinous) dorsal fins, and at least since Jurassic times the 

 Notidanidse have possessed only one ; in the last of the Hybodonts, 

 moreover, there are well-calcified vertebra?, while so advanced a 

 stage is far from reached in the living Notidanidas. I have likewise 

 been able to determine that the Wealden species of Hybodus only 

 possessed five branchial arches ; and there are also other points of 

 divergence to be taken into account. When, however, more is 

 known of the anatomy of the earliest Mesozoic forms, it may still 

 be possible to show that the evidence of the teeth is not altogether 

 misleading, but that even if the Notidanidaa are not an early offshoot 

 of the Hybodontidse, they are at least derived from the same primi- 

 tive stock. 



EXPLANATION OF PLATE VI. 



Fig. 1. Tooth commonly referred to Hybodus polyprion, Ag. Great Oolite, Bath. 

 B.M. (p. 2186.) 



,, 2. Tooth commonly referred to Hybodus polyprion, Ag. Great Oolite, Stones- 

 field. B.M. (35494.) 



„ 3. Notidanus eximius, Wagn. ; lower tooth. Corallian, Schnaitheim. B.M. 

 (35763.) 



1 Smith Woodward, " On the Relations of the Mandibular and Hyoid Arches in 

 a Cretaceous Shark (Hybodus dubrisiensis, Mackie)," read before the Zoological 

 Society, April 20th, 1886. 



