62 CESTRACIONTS. 



punctation of which is too minute for the naked eye ; the enamel is 

 disposed in a series of concentric lines around the base of the crown ; 

 these lines extend lower down on the posterior than on the ante- 

 rior part of the tooth ; and the enamel terminates on both sides in a 

 line which is convex towards the base of the tooth, contrariwise to 

 the terminal contour of the enamel in the compressed teeth of the 

 sharks. The osseous basis of the tooth terminates in an expanded 

 obtuse convex margin. This lamelliform tooth is bent slightly upon 

 itself so that a vertical section exhibits a slight sigmoid flexure (see 

 PL 22, figs. 3, 4, and 5.) 



In a second species of Petalodus ^Pet. serratus) , the trenchant mar- 

 gin of the tooth is more strongly serrated, and in a third species. 

 Pet. dentatus, it is notched or dentated.(l) 



The body of the tooth of the Petalodus is everywhere traversed 

 by medullary canals, which are fewer, relatively larger, and more 

 irregularly and reticulately disposed than in the teeth of Chomatodus, 

 or of any other genus of Cestraciont. The interspaces of the me- 

 dullary canals do not quite equal the diameter of the canals them- 

 selves : they are traversed by calcigerous tubes, as numerous and 

 minute as in the Psammodus, but similar in their wavy disposition 

 to those of the Acrodus. The short terminal branches of the medullary 

 canals, which distribute the calcigerous tubes to the enamel-like outer 

 layer, are slightly bent downwards, or towards the base of the 

 tooth. 



24. A very interesting modification of the teeth which resemble in 

 structure those of Psammodus, is presented in the extinct genus 

 Cochliodus of Agassiz. Here the jaws are paved with teeth arranged in 

 a few oblique contorted series, as in the Cestracion, but a single tooth 

 occupies the space covered by an entire row in the existing Australian 

 genus. In the specimen figured, in the collection of Capt. Jones, 

 R.N., there are three of these large contorted dental plates in each 

 ramus of the jaw, (PI. 22, fig. 1.) 



The microscopic structure of these large teeth closely resembles 

 that of the true Psammodus ; the medullary canals have the same 



(I) Both these species are founded on specimens in the beautiful collection of the fossils of 

 the mountain limestone, in the possession of Capt. Jones, R.N. M.P. 



