Structure and Classification of the Asterolepidse. 41 



densis (tah. et fig. cit., No. 17), and it is indicated, though, 

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

 Yig. 5). 



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 

 posterior dorso-lateral (see PL IL, 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 

 proportional extent of the proximal segment of each. Con- 

 sequently the articular and marginal plates of that segment 

 are of greater length than in those two genera ; but what is 

 more remarkable is, that the anconeal element (PL II., 

 rig. 4, a) is reduced to a small rounded plate on the dorsal, 

 and apparently entirely wanting on the ventral aspect of the 

 limb ; so that beyond the articulars the marginals are entirely 

 in contact with each other on the ventral side, and only 

 separated towards their extremities on the dorsal. In so far 

 as the proximal joint is concerned, the limb of Bothriolepis 

 may be said to be simpler in construction than in Pter- 

 ichthys ; but this is not true of the distal part, in which both 

 the central and marginal rows contain each at least one 

 additional plate. 



Tail. — It is remarkable that no tail is seen in Bothriolepis, 

 although numerous specimens both of B. canadensis and B. 

 hydrophilus seem perfect in every other respect. It is there- 

 fore perfectly plain that caudal scales were absent, though it 

 does not seem to me quite so safe to assume that no caudal 

 appendage was ever present ; for the posterior aspect of the 

 carapace shows a large opening just as in Pterichthys, out of 

 which it is difficult to conceive that absolutely no body- 

 prolongation ever proceeded, and it does seem quite possible 

 that a tail might have existed, though unprovided with hard 



