96 F. Chapman : 



Approximate length of Post-orbital Valley - - 18mm. 



Greatest longitudinal extent of series of Dorso-lateral 



scales represented in this specimen - - - 14mm. . 



Average diameter of tubercles on Head Shield - - 1.5mm. 



Average diameter of tuliercles on Dor.so-lateral scales Imui. 



Atiinities. — The present species shows certain affinities to the 

 three known species of Thyestes, viz., T. verrucosus, Eichwald ; 

 T. egertoni, Powrie and Lankester sp.,^and T. salteri, Egerton, sp.' 

 The outline of the head siiield, however, is not so long, pi-opor- 

 tionally, in any of the above-named species, our specimen being 

 more decidedly narrowed in front. 



The forward position of the occipital crest corresponds most 

 nearly with that in T. egertoni and T. salteri, T. verrucosus having 

 the crest prolonged far behind the points of the cornua. As 

 regards the post-orbital fossa, the border of its plate in T. 

 magniticus is regularly pyriform or Plorence-Hask shaped, rather 

 than elongately triangular, as in the restored figure of T. 

 verrucosus given by Rohon, ' whilst in T. egertoni it is apparently 

 elliptical. 



Although our specimen is not sufficiently well preserved to 

 allow one to speak of the actual form of the cornua, the base of 

 the left coriiu is so shaped that it seems probable, similarly with 

 T. salteri, they were more prolonged than in T. verrucosus, and 

 recurved towards the body as in T. egertoni, but were not so 

 slender. The tubercles of the head shield are of a uniform sizi- 

 in our species; whilst they are varial)le in T. verrucosus, and 

 comprise both large and small. 



The characters which help to confirm the above descriljed 

 species as belonging to the genus Thyestes are : — 



1. The pre.sence of fused posterior dorso-lateral scales. 



'1. The general form and outline of the head shield. 



3. The coarsely tuberculated surface of the head shield, and 

 posterior body scales, comparable to some extent with that of 

 Thyestes verrucosus. 



1 (Auchenaspis egertoni). " Fishes of the Old Red Sandstone." Pal. Soc. Moii., 1870, 

 p. 57, pi. xiii., fiifs. 'i-f> ; woodcut 8(1. 

 '2 CAuchenaspis salteri), Eg-erton. Quart. .Journ. Geol. Soc, vol. xiii., 1**57, p. 286, pi. ix. 

 ::; Op. cit., pi. 1, fig. 1. 



