VERTEBRATES. 365 



very small, form unknown, second tooth spirally enrolled, 

 narrow, obliquely triangular in outline, posterior margin round- 

 ed, lateral margins sulcated for co-adaptation, crown bearing 

 a single central revolving ridge ; third tooth much broader, 

 also strongly revolute, bearing a distinct ridge on its anterior 

 border and another much stronger on the median line ; a deep 

 and smooth sulcus separates the ridges. 



These teeth form a typical species of Cochliodus corresponding accurately in 

 position, relations, and general form with those of C. contortus, Ag., on which 

 the genus was founded. 



Deltodus had apparently a similar series of teeth on the mandibles, but they 

 were less convolute. There are, however, connecting links between these gen- 

 era which render it very difficult to separate them. 



In our description of Cochliodus nobilis (vol ii, p. 89) we noticed the discov- 

 ery of a mass of teeth, evidently the dentition of one individual, which includes 

 forms that have been referred by Agassiz to Cochliodus, Helodus, and Streblo- 

 dus. We then suggested that the teeth having the Helodus form were placed 

 in the middle and anterior portion of the jaws, corresponding to the middle 

 series of conical teeth in Cestracion. 



Prof. Owen has recently figured mandibles of Cochliodus and other Cestraci- 

 ontsin which no space is left between the convolute teeth for any such group as 

 those of Cestracion referred to. It should be stated, however, that the denti- 

 tion of the upper jaw of these fossil Cestracionts has never been seen in place, 

 and the mandibles of Cochliodus have never been found absolutely terminated 

 anteriorly. In Cestracion they approach each other so closely posterior to the 

 group of conical teeth, that if the extremity were removed by decay or vio- 

 lence, the jaw would seem to be normally terminated without any cuspidate 

 teeth. 



Hence, we may say that the presence of conical teeth in the dentition of any 

 of the fossil conchivorous sharks is not yet disproved.* 



Formation and locality : Burlington limestone ; Burlington, Iowa. 



*The figure given as that of the head of " Cestracion, Philipi " in Owen's Palaeontology, 2d 

 edition, p. 126, is really a representation of the head of Myliobates Aquila turned wrong side up. 



