JuLT 12, 1907] 



SCIENCE 



47 



the arrangement of the bones of the head-roof. 

 (2) That the living types of lung-fishes, par- 

 ticularly the Australian Neoceratodus, show 

 the closest affinities with Devonian arthro- 

 dires — especially with Mylostoma, the form 

 which has pavement-like dental plates. (3) 

 And that he has discovered the way in which 

 the dental plates of Mylostoma were originally 

 arranged. These theses may now be ex- 

 amined; but for convenience, they will be 

 taken up in an inverted order. 



I. As to the dental apparatus of Mylostoma. 



In various forms of arthrodires there were 

 present at least three pairs of dental 

 plates; — there was possibly a greater number 

 of these plates, in pairs or azygous, but the 

 proof is still imperfect. In the case of 

 Mylostoma, the three pairs of plates occur 

 in a single well-preserved specimen which was 

 first described by the reviewer {Mem. N. Y. 

 Acad. Sci., 1901), who endeavored to show 

 that these plates corresponded to the " pre- 

 maxillary," " maxillary " and " mandibular " 

 plates of other arthrodires, and that they 

 were arranged in the mouth in a similar 

 manner — the smallest plate, sharply triangu- 

 lar, becoming the " pre-maxillary," and the 

 medium sized, ovoidal one, the " maxillary." 

 In the fossil, moreover, the normal position 

 of the plates in the mouth indicated, 

 since two of the plates, " maxillary " and 

 " premaxillary," are preserved side by side, in 

 singularly perfect contact. These conditions, 

 then, become the point of departure for East- 

 man's detailed studies, which involve, by the 

 way, over two-score octavo pages. Thus: ta- 

 king a large series of detached dental plates 

 (which, we infer, may well have belonged to 

 different individuals, species and probably even 

 genera), Eastman places them together, secun- 

 dum artem, until their grinding surfaces fit, 

 and thus obtains their " true ariangement." 

 By this mode of procedure, he first of all changes 

 the relative position of the "maxillaries," as 

 given by Dean, and figures a pair of new ele- 

 ments, " vomerines," lying crosswise in the 

 front of the mouth. This arrangement, how- 

 ever, does not give permanent satisfaction to 

 its author, for in his third paper, the vomerine 



plates are withdrawn from the complex, and in 

 this process each of these elements is rotated 

 90°, changed sides, transferred from the upper 

 to the lower jaw, and described as having be- 

 longed to a new mylostomid. This change, 

 however, does not deter the author from still 

 insisting upon the actuality of vomerine plates. 

 On the evidence of a new arthrodire, Dino- 

 mylostoma, in which he describes three pairs 

 of dental plates, he argues, again from their 

 needs in fitting together, that there must have 

 been still another (i. e., a fourth) pair of plates. 

 Indeed, he declares confidently that, " unac- 

 quainted though we be with actual specimens, 

 the existence of vomerine teeth in Mylos- 

 toma, real or potential, is an assured fact " ! 

 That this may be so one will readily admit, 

 but it is not quite obvious from Eastman's 

 argument, especially when it entails the corol- 

 lary that the two well-known pairs of upper 

 dental plates of Mylostoma are the homologue 

 of the single pair of "shear teeth" of a closely 

 similar arthrodire (Dinichthys) . For we can 

 not understand why we should be asked to be- 

 lieve that two arthrodires, similar to each 

 other in a host of characters, should be so 

 distinct in this important particular? Nor 

 does it make the argument quite convincing 

 when Eastman points out that the " palatine " 

 plate in the young lung-fish, Neoceratodus, 

 passes through a stage in which it shows 

 traces of subdivision (= a " Mylostoma 

 stage"), for this implies a finished perfection 

 of the embryological record, which would 

 hardly have been assumed by even Haeckel in 

 his palmiest days. 



In short, I can not feel that the work of 

 Eastman on mylostomid dental plates is con- 

 vincing. He has not demonstrated that the 

 plates in Mylostoma were more numerous than 

 those well known in other arthrodires, nor has 

 he modified satisfactorily our views as to 

 their relative arrangement. The evidence 

 of the first specimen, which shows two of 

 the dental plates in closely fitted contact, is 

 still, I believe, better evidence in the matter 

 of mylostomid dentition than that obtained 

 by elaborate fittings of detached and pos- 

 sibly (bear witness Eastman's " vomerines ") 



