70 



T1II0 CHRONOLOGICAL 1MKTRI IUITION 



were in existence about ninety species ; during the Coal-measures, fifteen ; during the 

 Permian, only Jive. Thereafter they are not known. 



The Cochliodontidse formed an extensive family, with a dentition which reminds 

 one strongly of that of the modern Heterodontus (Cestracion) ; but this dentition was, in 

 a way, highly specialized; and it is quite improbable that it later became simplified into 

 the dentition of any post-pal ieozoic sharks. Moreover, this family too was a decadent 

 one after Subearboniferous times ; since out of more than one hundred and seventy-five 

 species which have been described, only fifteen or fewer belong to the Coal-measures. In 

 the Mississippi valley the family had its culmination in the Keokuk and St. Louis 

 epochs, the middle of the Subearboniferous. The whole thickness of the deposits of the 

 Coal-measures of the Mississippi valley has furnished only about ten species, about as 

 many as are found in the; Kinderhook, a formation about two hundred feet thick. It 

 may be further observed that not a single known genus of Ibis family came into exist- 

 ence during the Coal-measures period. 



This review of families brings us to a, group of genera which have been included by 

 Mr. A. S. Woodward in the family Cestraciontidai, or as the writer prefers to call it, 

 Heterodontidee. This family may, for convenience of discussion, be regarded as consist- 

 ing of a group of- Palaeozoic, or more specifically Carboniferous, genera and of a group 

 of post-palaeozoic genera. The former group, like most of the families which we have 

 so far considered, had its maximum of development, both as to genera and species, dur- 

 ing the Subearboniferous. Nearly all the genera too had their origin during Subearbo- 

 niferous times. Of this group Orodus may be taken as the type, being the best known, 

 most, abundant in species and most widely distributed. As in the case of the other 

 genera, of this group, all we know about Orodw is derived from its teetb ; and these 

 generally occur isolated. It is admitted by authors that these teeth are not greatly 

 different from those of Hylodus of post-paheozoie times. As defined and accepted, how- 

 ever, Orodw belongs essentially to the Subearboniferous period. Of thirty-eight 

 described species only one belongs to tin; Coal-measures, and. this to the lower division. 

 Of nineteen species described from the" rocks along the Mississippi river one belongs to 

 the lower Coal-measures, one to the; Chester, the uppermost division of the Subearbonif- 

 erous ; the remainder are nearly equally distributed among the other divisions. In Great 

 Britain the maximum of development of Orodus is in the lower portion of the Subear- 

 boniferous. Campodus has five species in the Coal -measures, two in the Subearboniferous. 

 Of the five species of Sphenacanthus two belong to the Subearboniferous, three to the 

 Lower Coal-measures. On the whole, the Palaeozoic contingent of the Heterodontidee 

 was a declining one from the time of the Subearboniferous period. 



The other members of Mr. Woodward's Cestraciontidse are found principally in the 



