ITS STRUCTURE, BULK, AND ASPECT. 101 



ciines that of its cranial buckler. The Pterichthys was formed 

 in nearly the same proportions. The Diplopterus was fully 

 seven times the length of its buckler ; and the Osteolepis from 

 six and a half to seven. In all the cranial bucklers of the 

 Asterolepis yet found, the snout is wanting. The very fine 

 specimen figured in page 71 (fig. 29) terminates abruptly at 

 the little plate between the eyes ; the specimen figured in 

 page 69 (fig. 28) terminates at the upper line of the eye. 

 The terminal portion which formed the snout is wanting in 

 both, and we thus lack the measure, or module^ as the archi- 

 tect might say, by which the proportions of the rest of the 

 creature were regulated. We can, however, very nearly ap- 

 proximate to it. A hyoid plate in my collection (fig. 51) is, 

 I find, so exactly proportioned in size to the cranial buckler 

 (fig. 29), that it might have belonged to the same individual ; 

 and by fitting it in its proper place, and then making the ne- 

 cessary allowance for the breadth of the nether jaw, which 

 swept two- thirds around it, and was surmounted by the snout, 

 we ascertain that the buckler, when entire, must have been. 



Fig. 51. 



a 



HTOro PLATE OF THURSO ASTEROLEPIS.* 



(One-half the nat. size, linear.) 



* The shaded plate (a), accidentally presented in this specimen, be- 

 longs to the upper part of the head. It is the posterior frontal plate F, 

 which half-encircled the eye-orbit (see fig. 31) ; and 1 have introduced 

 it into the print here, as in none of the other prints, or of my other spe- 

 cimens, is its upper surface shown. 



