SELACHIANS AND PYCNODONTS. 



By S. W. WILLISTON. 



The following notes and descriptions of selachian and 

 pycnodont teeth are based upon the material that has accumu- 

 lated in the University of Kansas Museum during the past ten 

 or twelve years, supplemented by a collection kindly loaned for 

 study by Mr. T. W. Stanton, of the National Museum. The 

 material is by no means exhaustive, nor even sufficient to settle 

 several doubtful points, but I trust that, incomplete as it may 

 be, it will be of service in the determination of our numerous 

 forms. 



"The specific determination of the detached teeth of sharks 

 and skates is little more than guesswork, and to decide upon 

 their generic relationships with any approach to certainty is 

 also often very difficult." 1 Nevertheless, because such de- 

 tached teeth are so often found, and connected series so rarely, 

 an attempt at their determination is desirable. Fortunately, 

 in the present collection there are several forms represented by 

 such complete specimens that the positive addition they afford 

 to the knowledge of the species and genera is very welcome. 



MYLIOBATIDiE. 



PTYCHODUS. 



Teeth with the crown more or less elevated and overhanging, 

 ornamented with transverse or radiating ridges, and surrounded 

 by a larger or smaller, finely marked area. Surface of root 

 smooth. 



This genus of Upper Cretaceous selachians was for a long 

 time placed among the cestracionts, but recent discoveries of 

 the nearly complete dentition render it more probable that its 



1. 'Woo'lwarr] : Proc. Geol. Assoc, XIII, p. 190. 



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