ATELEASPIS. 



2 95 



diameters. The same kind of unbranched canaliculi and terminal lacunae are 

 seen, often with great distinctness, in the basal layers. (Fig. 196, AL] 



It has been generally assumed that in the pteraspids true lacunae are absent. 

 It will be seen from the above account that they possess a primitive form of lacunae 

 that are similar to those in Limulus. They differ from typical bone lacunas in 

 their small size and in the absence of radiating canaliculi. 



Ateleaspis. 



In a specimen of Ateleaspis from Scaumenac Bay (Fig. 198), the shell in 

 tangential sections forms a continuous network of trabeculae. They are not as 



FIG. 199. .-1. Surface ornamentation of the thoracic shield of Limulus, near the lateral eyes. B. Same on the 

 haemal surface of one of the cornua. C. Cross-section of one of the cornua. 



FIG. 200. A, Surface view of semi-transparent exoskeleton of Limulus, from the flexible portion in the 

 olfactory region. It shows the peculiar groups of pore canals, lyng just below the surface, and radiating from the 

 base of the denticle-like spines; also the polygonal network of low ridges, or trabeculae, projecting from the 

 inner surface of the exoskeleton. B, Surface view of thoracic shield of an ostracoderm (Ateleaspis, sp?) showing 

 the polygonal areas, surface tubercles, and the peculiar grouping of canals lying just below the surface. C, Surface 

 ornamentation from the hasmal surface of thoracic shield of Limulus. D, Same, more highly magnified. 



distinctly laminated as in Pteraspis, although there is a dark brown axial core in 

 which most of the lacunae are located. The lacunae are spindle-shaped, with 

 more than one canaliculus, some of which can be traced to the outer surface of the 

 trabeculae. 



