NOTES ON CARBONIFEROUS COCHLIODONTS 23 



the jaw from their present positions on the slab shows that they are 

 badly disarranged and might easily come from two or more specimens. 

 He says 1 that of the two large teeth one belongs to the form called by 

 Davis posterior, and the other to the form called median, but that 

 they are not associated in such a way as to lend any support to that 

 author's theory as to their arrangement. But the specimen of P. 

 legrandensis furnishes positive evidence that Davis' theory in regard 

 to the linear arrangement of the teeth is correct, with certain modifica- 

 tions. Only two large teeth are present in Traquair's specimen, 

 while at least eight are necessary for the full dentition of both jaws. 

 The teeth are so associated that Traquair concludes that the large 

 plates occupied a position on the ramus similar to that of the median 

 teeth of Cochliodus, and that helodoid teeth were arranged in front 

 and external to them; but the specimen of Psephodus legrandensis 

 before mentioned shows that such was not the position of the large 

 plates, and that helodoid teeth could not have occupied the supposed 

 place with reference to them. 



Although the linear arrangement of the teeth of Psephodus postu- 

 lated by Davis is in part correct, the inner margins of the teeth are 

 placed outward in his restoration, as has been pointed out by Tra- 

 quair, 2 and a glance at the dentition of P. legrandensis (Plate I, 

 Fig. 2) shows that there was no place on the jaw for helodoid teeth. 

 On each ramus of the jaw one large posterior tooth, one median 

 tooth, usually much smaller, but in some species nearly as large as 

 the posterior tooth, and probably one small triangular anterior tooth, 

 are present. But instead of the teeth being arranged in a semi- 

 circle, as Davis supposed, those of the two rami touch or approach 

 each other very closely along the median line of the long axis of the 

 jaw. 



A comparison of the dentition of Psephodus, as shown in P. 

 legrandensis, with that of Cochliodus brings out certain striking 

 similarities. The arrangement of the large Cochliodus teeth is similar 

 to that of Psephodus, the principal difference being that they do not 

 meet along the median line, as in Psephodus, but spread apart, 

 leaving a V-shaped area. The specimen from which Newberry and 

 Worthen drew their conclusion that helodoid teeth were associated 

 1 Ibid., p. 342. a Ibid., p. 342. 



