164 



Contributions to Palaeontology. 



second annulation are slightly curved forward, but not 

 expanded. At the extremities of the posterior annula- 

 tion, and -separated from it by an indentation, there is 

 on each side a small node lying nearly in the line of the 

 dorsal furrow. 



Occipital furrow strongly marked, straight in the middle, 

 slightly curving forward at the extremities, and contin- 

 ued in the fixed cheeks : occipital ring narrow, prominent, 

 and rising above the convexity of the glabella. Dorsal 

 furrow strongly defined, slightly interrupted behind by 

 the little nodes before mentioned. The frontal limb is 

 depressed, a little convex in front of the furrow, and 

 bent abruptly downwards. 



The fixed cheeks rise abruptly from the frontal limb, to 

 a prominence continued from the palpebral lobe to the 

 dorsal furrow : the principal part of the area from the 

 eye backwards being depressed convex, and produced 

 into an angular termination at the extremity of the 

 posterior limb. Surface strongly striate. 



The surface characters are rarely seen in the casts ; but in the 

 impressions in the sandstone, they may be detected. The peculiar 

 form of the glabella and fixed cheeks, the anterior position of the 

 eyes, and strongly furrowed glabella with prominent rounded anterior 

 lobe, are characters which readily distinguish this species from any 

 other, except the young of Ptychaspis (Z>.) miniscaensis. 



Dr. Owen has given the following 



" Specific character. This species is distinguished particu- 

 " larly by the glabella and cheek-plates, as far as they 

 "are preserved, being studded with minute granules. 

 " The glabella is divided by three furrows into four seg- 

 " ments : the posterior segment terminates on either side 

 "in two obscure basal tubercles." 



Strictly this description does not apply to the specimens ; but I 

 suppose the occipital furrow to be one of the three furrows mentioned, 

 while the " posterior segment" terminating "in two obscure basal 

 tubercles" can apply to the slightly tuberculated extremities, and 



