Victorian Fossils, Part VII. 95 



It is probable tliat the tubercles are bi'ought out in stronger 

 relief by the removal of part of the external layer of the shield. 

 Occasionally the tubercles are hollow at their summits, a charac- 

 ter likewise observed in certain species of Cephalaspis.^ A more 

 or less divergent and quincuncial arrangement is observable with 

 regard to the tubercles, and they appear to radiate from near the 

 base of the head shield. There is an obscure areolation of the 

 surface of the shield, due to numerous sinuous ridge-like lines; each 

 areola enclosing several tubercles. The margins of the post-orbital 

 plate are distinctly seen, as well as the inter-obital ridge ; the 

 latter is tuberculated on the lateral slopes, rough on the summit, 

 and crenate in front. Position of eyes apparently indicated by 

 a pair of elliptical depressions, situated near the base of the 

 inter-orbital ridge, at a distance of about \ the length of the 

 shield, measured from the front. 



Dorso-lateral scales. — In the known species of Thyestes the 

 fused posterior body scales seem to have been contiiied to a single 

 series. In our specimen there appear to be some indications of 

 longitudinal junction lines, separating a dorsal and lateral series, 

 but the evidence for such is not so clear as to allow one to speak 

 positively. The lateral edges of the posterior scales extend almost 

 to the base of the cornua. 



The surface ornamentation of the dorsolateral scales is similar 

 to that on the head shield but liner, the tubercles being about | 

 the diameter of those on the head shield. The margins of the 

 dorsolateral scales are strongly scalloped, and in the present 

 specimen do not extend backward so far along the median dorsal 

 ridge as in Thyestes verrucosus, Eichwald.'^ 



Dimensions (Approximate, on account of some distortion): — 



Length of Head Shield along median line, from 



anterior rim to the crista occipitalis - - - 39mm. 



Width of Head Shield, measured from the widest 



part at the base of the cornua - - - - 88mm. 



Distance of the orbits from the anterior rim of the 



Head Shield, about ------ 16mni. 



1 See "Fishes of the Old Red Sandstone," Powrie and Lankester. Pal. Soc. Mon., 

 vol. xxiii., 1870, p. 55, pi. xiii., fig 19a (C. lightbodii). 



2 See Rohoii, .J. V., "Die obersilurischen Fische von Oesel I." Mem. Acad. Imp. Sci., 

 St. Petersburg, ser. 7, vol. xxxviii., 1892, pi. 1. 



