﻿176 OSTEOSTRACI. 



P. 2253, P. 2255. Two specimens, one in the form of a thick 

 quadrate plate ; Bush Pitch. Egerton Coll. 



P. 4478. Several fragments ; Bush Pitch. Ennislillen Coll. 



P. 5087. Coarse fragment ; Bush Pitch. 



Presented by Jolm Edward Lee, Esq., 1885. 



Order II. OSTEOSTRACI. 



Exoskeleton consisting of calcifications, partly with bone-cor- 

 puscles ; each plate comprising three superposed layers, the middle 

 layer solid, with a coarse reticulation of large vascular canals. 

 Dermal sense-organs leaving no impressions upon the exoskeleton. 

 Dorsal shield consisting of one principal piece, sometimes with a 

 separate mesial piece or fused body-scales posteriori)- : orbits close 

 together. Ventral shield simple, or replaced by polygonal calcifica- 

 tions. [Jaws never preserved.] Paired fins absent. 



Synopsis of Families. 



Surface of shield tuberculated ; interorbital 



piece fixed Cephalaspid^ (p. 176). 



Surface of shield finely punctate ; interorbital 



piece loose Tremataspld^: (p. 201). 



Family CEPHALASPIDiE. 



Shield rounded or tapering in front, abruptly truncated behind : 

 interorbital piece firmly fixed ; ornamentation consisting of more or 

 less numerous tuberculations. [The middle layer of the shield 

 sometimes produced postero-laterally into a pair of flexible expan- 

 sions (? opercula).] Dorso-lateral squamation consisting of series 

 of very deep and narrow, imbricating scales. 



Synopsis of Genera. 



I. Anterior dorsal body-scales not fused into a con- 

 tinuous plate. 

 Postero-lateral angles of shield produced into 

 acute cornua not exceeding the shield in 

 length Cephalaspis (p. ] 77). 



