Extinct Selachian Asteracanthus. 339 



the posterior than the anterior extremity. Of the teeth of 

 series 2 five remain upon the left side and six upon the right. 

 Each of these measures 0*037 in length and differs from a 

 corresponding tooth of series 1 in its relatively less elevation 

 and the almost complete absence of the longitudinal keel upon 

 the crown. The teeth assigned to series 3 are very different 

 from the foregoing; they measure 0-04 in length, and six of 

 the left side and five of the right are available for study. 

 The anterior extremity is still less truncated than the poste- 

 rior ; but the contour of the low crown is only gently sinu- 

 ous, the posterior end being slightly upturned, and a more or 

 less rounded elevation occurring somewhat in advance of the 

 middle of the tooth. Series 4 is only preserved upon the 

 left side, where it is represented by seven teeth, each mea- 

 suring 0-048 in length, and differing from those of series 3 in 

 the considerable forward flexure at the point of maximum 

 coronal elevation. To series 5, 6, or 7, either of this or the 

 opposing dentition, may also be referred two small elliptical 

 teeth, of which one is shown in PI. XII. fig. 4. 



The five symmetrical median teeth assigned to the sym- 

 physis of the mandible have a much elevated, arched, longi- 

 tudinally-keeled crown, and measure 0'023 in the longer 

 transverse diameter. Those of series 1 only differ from the 

 median teeth in their unsymmetrical character, being very 

 similar to the corresponding opposing teeth ; they measure 

 0*024 in length, and six of each side are preserved in addition 

 to one incomplete germ on the right. The supposed teeth of 

 series 2 are perplexing on account of their number, which at 

 first sight leads to tlie suspicion of an error. No less than 

 twelve teeth of each side can be identified, in addition to a 

 germ on the right ; and this number is equalled by no other 

 known Cestraciont except Cestracion. Each tooth measures 

 0*029 in length and chiefly differs from a tooth of the oppos- 

 ing series 1 in its less elevation and the generally more trun- 

 cated character of the posterior extremity. The teeth of 

 series 3 much resemble those of series 2 of the upper jaw, but 

 are relatively shorter, measuring only 0*034 in length j and 

 of these four remain upon the left and six upon the right. 

 The teeth assigned to series 4 are only preserved, to the 

 number of five, upon the left ; and their difference from those 

 of the homologous upper series consists merely in their nar- 

 rowness and great relative length, the latter measurement 

 being at least 0*053. 



Dorsal Fm-sphies. — The two dorsal fin-spines associated 

 with the dentition just described are shown, of two thirds the 

 natural size, in PI Xil. figs. 5, 6. They are nearly of equal 



