Structure and Classification of the Asterolepidee. 499 



I have not seen the small median plate which Whiteaves 

 (same figure, no. 18) represents immediately behind the two 

 last mentioned, and concerning which he remarks, " Judging 

 by analogies with the Asterolepis of Hugh Miller, but not of 

 Pander, this may have been the hyoid plate." Unfortu- 

 nately for this comparison, the " hyoid " plate of Hugh Millei-'s 

 Asfci-olejns [^Homosteus] was, thirty years ago, determined 

 to be the median dorsal plate of its carapace (7, p. 76). 



Body-carapace. — This is more depressed in Bothriolepis 

 than in PterichthySj has a dorso-lateral angulated margin as 

 well as a ventro-lateral one, and the dorsal surface is broader 

 than the ventral one. The median dorsal plates are not so 

 acutely elevated mesially as in Pterichthys ; in some species 

 they are only gently convex on the upper aspect. The ante- 

 rior median dorsal, usually rather wide in its shape, articu- 

 lates as in Pterichthys (but not as in Asterolepis)^ its antero- 

 lateral margin overlapping the anterior dorso-lateral, while 

 the postero-lateral margin is, on the other hand, overlapped 

 by the posterior dorso-lateral. The inner surface of this 

 anterior median dorsal (PI. XVIII. fig. 3) shows a sharp 

 median ridge, from which anteriorly two short branches are 

 seen to diverge at acute angles forwards and outwards. On 

 the inferior surface of the body the anterior ventro-laterals 

 (PI. XVIII. fig. 5) show a peculiarity in shape which dis- 

 tinguishes them from the corresponding plates in Pterichthys 

 and Asterolepis in not exhibiting in front the prominent 

 emargination for the semilunar plates seen in those genera. 

 In fact no precisely similar semilunar plates exist, though 

 these seem to be represented by a single small triangular one 

 occupying the median notcii at the union of the two anterior 

 ventro-laterals. This is figured by Whiteaves in B. cana- 

 densis [tab. et Jig. cit. no. IV), and it is indicated, though 

 obscurely, in many specimens of B. hydrophilus (PL XVIII. 



The lateral-line groove is continued on the body-carapace 

 from the external occipital along the dorso-lateral plates on 

 each side immediately below their longitudinal flexure. In 

 addition' to this another groove in the form of an inverted V 

 is seen on the dorsal surface, the apex of the V being a little 

 in front of the middle of the anterior median dorsal plate, 

 while its legs extend outwards and backwards over the poste- 

 rior dorso-lateral (see PI. XVIII. fig. 4). 



Arms. — The pectoral limbs in Bothriolepis are distin- 

 guished from those of both Pterichthys and Asterolepis by 

 their greater length, which usually equals or even exceeds 

 that of the carapace, and this is due chiefly to the greater 



35* 



