EASTMAN: STRUCTURE AND RELATIONS OF MYLOSTOMA. 21 



cumstances it is not to be wondered that his descriptions were imper- 

 fect, and his ideas as to the systematic position of the genus confused. 

 Newberry's original suggestion was that Mylostoma should be referred 

 "to the group which includes Dipterus, Palaedaphus, Ctenodus, and 

 Ceratodus." 1 Six years later he included the genus under the head of 

 " Placoderms," considering it to be an extremely specialized " member 

 of the family of the Dinichthidae ; " 2 yet on another page of the same 

 work these fishes were denned as " Dipterine Ganoids of large size," 

 and the points of resemblance between them and Ctenodipterines were 

 considered " sufficient to justify the inference that they were all 

 related." 3 



Without attempting a theoretical reconstruction of the Mylostomid 

 type of dentition, Newberry was nevertheless convinced that it was 

 extremely complicated. The beveled edges and other appearances of 

 certain specimens were interpreted by him as indicating co-adaptation 

 with contiguous elements, whence it followed that several pairs of 

 dental plates must have been present in the lower jaw, and the upper 

 dentition was supposed to be in the form of a " tesselated pavement 

 consisting of many pairs of plates." 4 The author's own words with 

 reference to the latter point are as follows : — 



" The dental plates of the upper jaw form several pairs, of which the central and 

 largest are rudely triangular in outline, with a flattened or concave triturating 

 surface, bearing, as do some of the inferior teeth, evidences of wear. The surface 

 of attachment to the cranium of these dental plates is flat or concave and 

 somewhat rough, from the coarse cellular tissue of the bone ; the sides are straight 

 or beveled, apparently for cfr-adaptation,and by this character favor the conclusion 

 that the dentition consisted of many pairs of plates, constituting a tesselated 

 pavement ; the crowns of the teeth below being convex, those above concave." 



It was reserved for Dr. Bashford Dean 5 in 1893, and more completely 

 in 1901, to disclose the essential characters of Mylostoma, and to 

 demonstrate its close relation with Dinichthys, as the result of his 

 study of an admirably preserved specimen of M. variabile from the 

 Cleveland shale of Ohio. Eventually, one of the counterparts of this 



1 Newberry, J. S. Some interesting remains of fossil fishes, recently dis- 

 covered. Trans. N. Y. Acad. Sci., 1883, 2, p. 146. 



2 Idem., Monogr. U. S. Geol. Surv., 1889, 16, p. 163. 



3 Ibid., pp. 161, 163. 4 Ibid., p. 165. 



5 Dean, B. On Trachosteus and Mylostoma, Notes on their Structural Char- 

 acters. Abstract. Trans. N. Y. Acad. Sci., 1893, 12, p. 70-71. Palaxmtological 

 Notes: On the Characters of Mylostoma Newberry. Mem. N. Y. Acad. Sci., 

 1901, 2, p. 101-109. 



