SUB-CLASS II 



OSTRACODERMI 



55 



bony plates, and seems to represent an operculum. Body triangular in trans- 

 verse section, the flanks covered with three longitudinal series of deepened 

 scales, the middle series deepest. Ventral scales small. Dorsal fin triangular, 

 at the base of the heterocercal tail ; membrane of dorsal and anal fins stiffened 

 with very small calcifications which are 

 arranged in parallel rows simulating rays. 

 C. lyelli, Ag. (Fig. 106), from Lower Old 

 Eed Sandstone of Scotland and west of 

 England. C. magnifica, Traq., the largest 

 known species, with shield measuring 0"22 

 m. across, from Caithness Flagstones. 

 Other species from Upper Silurian and 

 Lower Devonian of Scotland, England, and 

 Canada. One species (C. loMceps, Traq.), from 

 Upper Devonian, Scaumenac Bay, Canada. 



Eukeraspis, Lank. ' (Sderodus, Pleetrodus, 

 Ag.). As above, but posterior cornua 

 excessively elongated, and a row of twelve 

 large "marginal cells" around the rim in 

 front. The detached denticulated cornua 

 were originally mistaken for jaws. E. 

 pustulijera, Ag. sp., from Upper Silurian, 

 Herefordshire. 



Thyestes, Eichw. (Auchenaspis, Egerton). Head shield as in Cephalaspis, but 

 three or four series of dorso-lateral scales fused into a continuous plate 

 immediately behind it. Body depressed, ovoid in transverse section. Known 

 species very small. T. verrucosus, Eichw., from Upper Silurian, Isle of Oesel. 

 So-called Auchenaspis from Upper Silurian and Lower Old Red Sandstone 

 Passage Beds of Herefordshire. 



Head shield of Cephalasjns lyelli, Ag. Lower 

 Old Red Sandstone ; Herefordshire (after Lau- 

 kester). i/o. 



Family 3. Tremataspidae. Smith Woodward. 



Head shield rounded or tapering in front, fused with the dorsal tody-shield, which is 

 truncated behind; interorhital piece not fixed. Upper Silurian and Lower Devonian. 



Trematasjns, Schmidt. Dorsal shield rounded in front, gently arched, 

 without cornua ; ventral shield simple. External surface covered with 

 punctate ganoine, the punctations often arranged in reticulating lines ; 

 superficial tuberculations almost or entirely absent. Two pairs of large 

 openings laterally placed in the anterior half of the dorsal shield are in- 

 terpreted by Smith Woodward as occurring in the roof of the branchial 

 chamber, covered over during life but ensuring flexibility. Ventral aspect 

 immediately behind the position of the mouth covered with a number of 

 symmetrically arranged polygonal plates, between which and the great ventral 

 plate of the trunk is a transverse series of nine small openings on each side, 

 probably of branchial nature. Tail covered with rhombic and polygonal scales, 

 which have been named Dasylepis, Dictyolepiis, Melittomalepis, and Sfigmolepis 

 by Pander. T. schrenki, Pander sp., and other species in the Upper Silurian, 

 Isle of Oesel, Baltic Sea. 



Didymaspis, Lankester. D. gxindrodi, Lank., from Lower Old Red Sand- 

 stone, Ledbury, Herefordshire. 



