EASTMAN: THE DINICHTHYIDS. 21 



gest and most perfect plate that the writer has seen is preserved iu the 

 Museum of Comparative Zoology, and measures 65 cm. in length (Cata- 

 logue No. 1325). The corresponding element in D. intermedius is hardly 

 to be distinguished except for its smaller size. An excellent example of 

 the latter species belonging to the School of Mines Cabinet of Columbia 

 University shows the postero-dorso-laterals of either side of the body 

 commingled with other plates pertaining to the same individual ; it is 

 valuable for furnishing comparative measurements of the different bones, 

 and deserves further study. 



The orientation of the plate in question may be readily determined, 

 either by an inspection of the overlapped area, or by noting the course 

 of the sensory canals. These arise at the anterior border, where they 

 meet the single straight furrow that traverses the antero-dorsal-lateral ; 

 and from this point they sweep inwardly, sometimes as a single and 

 sometimes as a double channel, as far as about the middle of the exposed 

 portion of the plate, where they cease. In this respect the genus dilfers 

 from Coccosteus, which has the canals continued on to, and in some 

 cases entirely across, the dorso-median. The insunken area formed by 

 the overlap of the latter plate stands in marked contrast to the irregular 

 depressions produced by the overlap of the antero-dorso-lateral. The 

 graceful curve forming the posterolateral boundary of the dorso-median 

 is projected upon the underlying plate, and shallow depressions are left 

 where the transverse ridge on the under surface of the dorso-median 

 rested on the subjacent plate. This ridge, it should be noted, occupies 

 the same relative position as its homologue in Coccosteus. 



The upper boundary of the lateral plates is indicated by a deeply in- 

 sunken area on the antero-dorso-lateral, and a slight indentation on the 

 free margin of the postero-dorso-lateral. Below, these as yet undiscov- 

 ered plates must have been connected with the ventral armoring, either 

 directly, or more probably through the intervention of the " claviculars." 

 The curvature of the ascending arm of the latter furnishes us at the same 

 time with the curvature of the missing laterals, and we can also form 

 an approximate estimate of their height and length. It is to be hoped 

 that the laterals may yet be identified as such, when the entire dermal 

 covering of Din ichthys can be compared plate for plate with its European 

 congeners. 



Ventral Plates. — Grave difficulties have been encountered in the 

 attempt to reconstruct the ventral armor of Dinichthys, owing to the 

 detached condition in which the plates have invariably been found. It is 

 perhaps hut natural that the views which were originally entertained 



