EASTMAN: MYLOSTOMID DENTITION. 225 



splenials. Functional surface moderately convex, of narrow, subtriangular outline, 

 tapering and acutely pointed in front, witii elongated linear inner margin showing 

 distinct evidence of coadaptation along the median line, and notched posteriorly in 

 the vicinity of the single rounded eminence into which the ridge along the inner 

 margin gradually rises. Adjacent to this eminence are to be seen in unworn 

 specimens a few rows of small rounded tubercles, radiating outwards, but tending 

 to become obliterated by use, and the larger emhicnce tending also to become 

 worn down. Oral surface sloping gradually towards the bevelled external margin, 

 but foiling precipitately along the inner and posterior borders. Under surface 

 displaying the characteristic ridges and depressions of Mylostomids, besides the 

 usual groove for lodging remnants of the Meckelian cartilage. Jaw-angle acute, 

 indicating a very slender, possibly anguilliform body. 



Homologies op Mylostomid Dental Plates. — Throughout the fore- 

 going section we have employed the term " palatal plates " merely as a 

 convenient designation for the paired elements forming the upper pave- 

 ment dentition of Mylostomids, without having precisely indicated their 

 relations to other forms of crushing dentition. Newberry may possibly 

 have assumed, although his writings nowhere indicate it, that these 

 plates are homologous with the dental elements in the upper jaw of 

 Dinichthys and other Arthrodires. He did, however, point out their 

 obvious resemblance to the single pair of upper dental plates in Cerato- 

 donts, his remarks on this subject being as follows : ^ 



" The resemblance of the teeth which I have supposed formed the roof of 

 the raouth to those of Ceratodus will strike any one who examines them, and 

 no closer analogy suggests itself in the whole range of ichtliyic dentition. 

 There is, however, this marked difference, that while in Ceratodus there is 

 only one pair of dentary [i. e., dental] plates borne on the palato-pterygoid 

 bones, in Mijlostoma there were certainly several pairs of pavement teeth in 

 the roof of the mouth. The spatulate bones which form the supports of the 

 principal dental plates of the lower jaw evidently represent the thin, flat- 

 tened, smooth, and once buried posterior end of the dentary bone \=. splenial] 

 in all of the Dinichthidae." 



The earliest effort to trace homologies between the dentition of 

 Mylostomaand Dinichthys is that of Bashford Dean, in 1901. Relying 

 upon analogy with Dinichthys, he assumed that the upper dentition of 

 Mylostoma must have been limited to two pairs of plates only ; and 

 finding that number to be present in the nearly complete example 

 of M. variahile studied by him, he concluded that one of these pairs 

 must correspond to the so-called " premaxillary " element, and the 

 other to the so-called " maxillary " or " shear-tooth " of Dinichthys. 

 Apparently the possibility did not occur to him that the two pairs of 



1 Loc. cit. (1889), p. 162. 

 VOL. L. — No. 7 15 



