No. 14. — Mylostomid Palatal Dental Plates. By C. R. 

 Eastman. 



The publication (Mem. N. Y. Acad. Sci., 1901, 2, pt. 3) of an import- 

 ant article by Dr. Baslifurd Dean " On the characters of Mylostoma 

 Newberry " marked a signal advance in our knowledge of this genus 

 of Arthrodires. The thesis referred to possesses the further distinction 

 of having made known for the first time nearly all the hard parts, in- 

 cluding the palatal and mandibular dental plates, of a single individual. 



Dean's theoretical restoration of the dentition in the example studied 

 by him, in which the component parts were slightly disarranged, was 

 accepted as valid for the next four or five years, when another tolerably 

 complete individual, the type of which is preserved in the Peabody 

 Museum at Yale University was described under the name of Dinomy- 

 lostoma heecheri (Bull. M. C. Z., 1906, 50, p. 23). In the light of new 

 evidence furnished by this second unusually perfect individual. Dean's 

 interpretation of the upper dental pavement of Mylostoma was modified 

 in several respects, notably as regards the presence of true vomerine 

 teeth (corresponding to the so-called " premaxillaries " of Dinichthys) in 

 the upper jaw, anterior to the two pairs of palato-pterygoid crushing 

 plates. 



Some discussion has taken place during the last three years as to the 

 correctness of the two extant interpretations of Mylostomid dentition. 

 Dean's view is supported chiefly by himself, but in part also by Dr. L. 

 Hussakof, in several articles that have appeared in Science, 1907-1908, 

 and in the publications of the American Museum of Natural History. 

 For a recent formulation of the newer hypothesis it will suffice to refer 

 to the memoir on Devonian Fishes, published in volume 18 of the Iowa 

 Geological Survey Reports. 



As matters now stand, it is a demonstrated fact that Mylostomids are 

 characterized by the possession of two pairs of tritoral pavement plates 

 in the upper jaw, and that at least one genus, Dinomylostoraa, is pro- 

 vided with still another, somewhat smaller pair of plates in advance of 

 these, making a total number of three pairs of upper dental plates in all. 



