June 25, 1909] 



SCIENCE 



997 



Oscar A. Johannsen, assistant professor of 

 civil engineering at Cornell University and 

 author of researches on the biology of water 

 supply, has accepted the professorship of ento- 

 mology in the "University of Maine. 



Dr. J. E. KiRKVFOOD, formerly an investi- 

 gator vpith the Continental-Mexican Rubber 

 Company in Mexico, has been appointed as- 

 sistant professor of forestry and botany in the 

 University of Montana. 



Chas. H. Taylor, a graduate student in 

 geology at the University of Chicago, has been 

 appointed assistant professor of geology in the 

 University of Oklahoma. 



Professor H. A. Wilson, F.E.S., of King's 

 College, London, has accepted the appoint- 

 ment of professor of physics in McGill Uni- 

 versity. 



Dr. Arthur Lapworth has been appointed 

 a senior lecturer in chemistry at the Univer- 

 sity of Manchester. He is the son of Dr. 

 Lapworth, F.R.S., professor of geology at 

 Birmingham, and is at present head of the 

 chemical department of the Goldsmiths' Insti- 

 tute, New-cross. 



DISCUSSION AND CORRESPONDENCE 

 MYLOSTOMro DENT-^i PLATES 



In a recent contribution by Dr. L. Hussakof 

 on "Relationships of Anaerican Arthrodires" 

 (^Bull. Amer. Mus. Nat. Hist, 26, art. 20), a 

 peculiar dental element is made known under 

 the caption of Dinognathus ferox. The desig- 

 nation applies to a supposed new genus and 

 species of Arthrodires, of doubtful family re- 

 lations, and whose characters are imperfectly 

 definable. The position of the plate in the 

 mouth is held' to be indeterminate, although 

 remark is made that "its form is not sug- 

 gestive of having been set in a titanichthid 

 mandible." 



Knowledge of this unique structure is the 

 more welcome, since, as the present writer be- 

 lieves, it dispels the mystery of the missing 

 upper dentition of Mylostoma terrelli Newb. 

 That the peculiar plate in question belongs to 

 the same sort of creature, if not indeed to the 

 identical species as that established by New- 



berry upon the evidence of a solitary man- 

 dibular plate (now the property of the Museum 

 of Comparative Zoology), seems practically 

 certain. At all events it can be provisionally 

 associated with the type of M. terrelli with the 

 same confidence that actuated Newberry's the- 

 oretical correlation of upper and lower dental 

 plates of M. variahile — an hypothesis after- 

 wards confirmed beyond peradventure by Bash- 

 ford Dean. 



The mylostomid nature of the novel dental 

 plate under discussion is unmistakable, one 

 might even say self-evident, the moment it is 

 perceived to be a eompoimd instead of simple 

 element, representing in its entirety the for 

 wardly placed pair of palato-pterygoid dental 

 plates common to Arthrodires and Ctenodip- 

 terines. Among the latter, Heliodus lesleyi 

 furnishes an analogous instance of fusion of 

 the corresponding parts. 



The newly discovered dental plate is signifi- 

 cant for yet another reason, namely, for en- 

 lightening us as to the extent to which the 

 components of the upper dentition of Arthro- 

 dires are capable of fusion inter se. Certainly 

 in Dinomylosioma, and presumably, also, in 

 Mylostoma proper, there is a single pair of 

 vomerine, and two distinct pairs of palato- 

 pterygoid dental plates. In Dinichthyids, so 

 far as known, the two last-named pairs on 

 either side are fused into a single " maxillary " 

 element or " shear-tooth." Dinognathus is 

 peculiar in having the forward pair of palato- 

 pterygoid tritors fused into a single plate, 

 whose periphery accords fairly well with the 

 antero-extemal contour of the mandible — that 

 is, on the assumption that Newberry's so-called 

 M. terrelli and the newly described Dino- 

 gnathus ferox relate merely to different parts 

 of the same dental apparatus. It is pertinent 

 to observe further that a rounded eminence 

 occurs in the median line anteriorly, as shown 

 in Dr. Hussakof's figure, corresponding in 

 size and position to surface elevations of the 

 homologous plates in M. variabile. The man- 

 ner in which these eminences interact with , 

 depressions in the functional surface of the 

 lower dentition has been discussed elsewhere. 

 As to the occurrence of vomerine teeth and a 



