THE UPPER SILTTEIAN OF NORTH AMERICA. 



51 



Prof. Huxley in tlie Quarterly Journal (1858). Prof. Lankester 

 has added little upon this subject*. 



Prof. Huxley describes the shield of Pterasjns {Scapliaspis) Lloydn 



as consisting of four distinct layers, which he describes thus {I. c. 



p. 275) :— ^ ^ 



" The total thickness of the section is about ^q of an inch, and 



of this amount about y|^- of an inch is occuiDied by the inner layer. 



Pig. I,— Vertical Section of the Shield o/Pteraspis. 



(Magnified 100 diameters.) 



[Copied from Prof. Huxley's figure, Q. J. G-. S. vol. xiv. pi. xv. fig. 1.] 



a. " Enamel "-ridges forming the outer layer. b. Eeticular layer. 



c, d. Middle and inner substances. 



e. Cavity filled with matrix, one of the supposed " ossicles." 

 /. Vascular canal. g. Matrix. 



* Prof. Huxley's first figure is referred to in his paper as of Pteraspis {Sea- 

 phaspis) Lloydii, but Prof. Lankester in reproducing it, writes of it, probably by 

 o-versight, as Fteraspis rostratus. 



e2 



