58 TRANSACTIONS OF THE [Xov. 16, 



Clark, been acquired by the paleontological museum of Coliunbia 

 Universit3^ 



The notes which were then published upon the relations of 

 the abdominal plates have now, however, to be reconsidered in 

 the light of the discovery- of a plastron of a Dinichthyid 

 (Dinicht/iys mino?-?) in which the elements are retained in 

 their relative positions. This specimen had been briefly de- 

 scribed during the past summer at the Buffalo meeting of the 

 Geologists,* by Mr. C. R. Eastman, of the Museum of Com- 

 parative Zoology at Harvard. The fossil has not as yet been 

 seen by the present writer, but, thanks to the notes and a 

 drawing which Mr. Eastman has kindly furnished, there seemed 

 to be little reasonable doubt that a definite knowledge of the rela- 

 tions of the elements of the plastron of Dinichthys had at lengtii 

 been determined. To the account of the ventral plates given 

 by Prof. A. A. Wright (Am. Geologist, Nov., 1894), the present 

 writer had in his paper added a spatulate or ensiform ele- 

 ment, PI. I, VM, which, he believed, corresponded to the two- 

 ventromedian plates (viz., antero-ventro-median and postero- 

 ventro-median) of Coccosteus, as shown, for example, in Tra- 

 quair's restoration."}" But in the newly discovered fossil the 

 ventro-median plates were distinctly separate, very much as in 

 Coccosteus, and the writer could not help feeling at once con- 

 vinced that the ensiform element he had described was in reality 

 double, the two separate plates having Ijeen preserved in their 

 normal condition of overlap, but the anterior one having become 

 so badl}' weathered that in view of later evidence, it could 

 not be believed to have been fused with the hinder plate. 



Mr. Eastman, in a review of the writer's paper,;}; had indeed 

 suggested this very reasonable correction, regarding the evi- 

 dence of Dr. Clark's fossil, as " not entirely' decisive." although 

 willing to admit that the median ventral plates in Dinichthys 

 might readily have become fused " as a device for strengthening 

 an otherwise fragile plastron." Such a device, we might conclude, 

 would be therefore a strictly adaptive one, and one which could 

 only from the evidence of Mr. Eastman's fossil reasonably be 

 looked upon as occurring in particular species of Dinichthys. But 

 certainly such a varintion in an element of the plastron would be 

 very much opposed to our deep-rooted ideas as to the degree to 

 which (among adult fishes)important dermal bones may fuse witii- 

 in the limits of a single genus. In other words, these ideas could 

 lead us to expect that the median ventral plates of Dinichthys 

 would either be separate or fused in all species. In view of the 



*Cf. Am. Geologist, Vol. XVIII., Oct.. 18Vi(i, p. 222. 

 + Ann. Mag. Nat. Hist. [6], Vol. V., (1890), pp. 125-136. 

 I Am. Geologist, Nov. 18%, p. ai7. 



