24 PROCEEDINGS OF THE 



occipital region like those already noticed by Cope and others in 

 the Pleuracanth sknll. There is thus confirmation of the view 

 that cranial cartilage sometimes became differentiated into separate 

 parts before it was covered with membrane-bones. 



Bll.VnYOUONTI. 



Among the earliest Elasmobranch fishes, there were many in 

 which tlie teeth did not fall away from the edge of the mouth 

 vvhen they were discarded, as in all modern sharks and skates, but 

 became fused with their successors into antero-posterior rows 

 which were retained in some way outside or beneath the teeth 

 actually in use. These rows, when completely i)reserved, exhibit 

 all the successive teeth acquired during the lifetime of the indi- 

 viduals to which they belonged ; and from them we learn that 

 man)' of the Palseozoic sharks (e.g., C'ampodus and NeUcoprion) 

 had successional teeth as numerous and as rapidly-growing as in 

 the familiar existing forms. Soine rows, however, such as those 

 of Jaiiassa and the Cochliodonts, prove that there were also other 

 sharks or skates in which the successional teeth were very few — 

 not more than 7 or S durinji: the whole lifetime and each one 

 much larger than its predecessor. We know very little of these 

 fishes beyond their teeth, but they seem to form a natural group 

 intermediate between the priuiitive Elasmobranchs and the 

 Cliima?roids or Holucephali. From the slowness of their tooth- 

 changes they may be named Bradyodonti. 



The tyi)ical family of the Bradyodonts is that; of the Petalodon- 

 t'.dae, represei'.ted by several Carboniferous genera but best kno\\ n 

 by the Lower Permian Jcaiassa*. This is a skate-shaped fish, with 

 paired fins which seem to have been on the ordinary Selachian 

 plan. The teeth are arranged in 5 or 7 antero-posterior scries, 

 forming a powerful grinding pavement, chiefly on the symphysis, 

 wliere the two rami at least of the lower jaw are firmly fused 

 together. Only the latest row of teeth is in use at any time, the 

 predecessors in the several antero-posterior series being piled up 

 beneath the functional teeth to act as supports (fig. 3). From 

 these piles we learn that during the greater part of its lifetime 

 each individual Janassa liad only 7 or 8 successional teetli. In 

 the Lower Carboniferous Climaxodus f, the teeth from early 

 youth to old age are spread in antero-posterior series as a pave- 

 ment along a continually elongating symphysis of the jaw, and 

 there are shown to be only 5 or 6 teeth in succession during the 

 greater part of the individual life. Many of these teeth exhibit 

 a conspicuous patch of much-hardened tubular dentine, very 

 suggestive of the tritors on the dental plates of the Chima-roids. 



The Carboniferous Psammodontidu' are known solely by the 

 teeth of Psammoih'S, which are flattened grinders arranged in one 



* O. Jaekcl, Zeitsclir. deutsch. geol. Gesell. vol. li. (1899), p. 2.59, pis. 14, lo. 

 t A. S. Woodward, Quart. Jor.rn. Geol. Soc. vol. Ixxv. (1920), p. 1, pi. 1. 



