BOTHRIOLEPIS. HI 



numerous entire specimens in our great museums have furnished us pretty well 

 with the means of becoming acquainted with its details. Its first describer, 

 Whiteavcs, has already given us a restoration 1 which affords an excellent general 

 idea of its shape and of the arrangement of its osseous plates, though faulty as to a 

 few details of the head, to which allusion has already been made. 



The head occupies nearly one third of the entire length and shows, on the upper 

 surface, an orbit which is smaller and further back than in Asterolepis. The median 

 occipital plate (m. occ.) has its lateral margin more perpendicular to the posterior 

 one than in the last named genus ; its anterior aspect shows, not merely a shallow 

 re-entering angle for the post-median plate, but a deep semi-elliptical notch or 

 excavation, on each side of which it takes part in the formation of the posterior 

 boundary of the orbit. Consequently the post-media h ( />. m.) is small, entirely received 

 in the aforesaid notch of the median occipital, and thus excluded from joining 

 the laterals as in PtericJttJu/s and Asterolepis. The lateral occipitals (/. occ.) and the 

 angular (ag.) do not call for any special comment, but the laterals (/.) are much 

 broader than in Asterolepis, while the extra-laterals (e. I.) are very small, narrow, 

 and pointed in front. 



The orbit is, as already mentioned, small compared with that of Asterolepis, 

 and, moreover, its anterior margin shows scarcely any re-entering flexure. Its 

 right and left portions are filled up by the ocular plates (p.), between which arc 

 other two, namely the median or pineal (m.), and a very narrow plate x close in 

 front of it. From the centre of this narrow plate, as shown by Whiteaves, a small 

 linear process with expanded lower extremity passes down perpendicularly into 

 the interior of the head, though what its function can be is difficult to imagine. 



On the under surface of the head (Fig. 58) and close to the anterior margin 

 are the two small maxillary plates (mx.), which differ in form from those of 

 Pterichthys (see p. C>5), firstly in having the external notch very shallow, and 

 situated on the anterior, instead of the posterior external angle of the plate, and 

 secondly, in having the posterior internal angle so rounded off that at the 

 symphysis the two plates touch each other only quite in front. These plates, as 

 in Pterichthys, are when visible almost always displaced. 



The pattern of the cephalic lateral line grooves is considerably different from 

 that in Asterolepis and Pterichthys. No transverse commissure unites the lateral 

 groove of each side across the occipital plates ; but in front, just at its incurved 

 flexure on the lateral plate, a conspicuous branch is given off, which runs forwards 

 and outwards to the margin of the shield, while immediately behind the origin of 

 this branch and on the inner side of the main groove a small ear-shaped mark is 

 often, though not always, seen. On the median occipital two slighter grooves are 

 observable, forming an angle with each other behind, whence diverging obliquely 



1 ' Trans. Koy. Soc. Canada,' vol. iv, sect, iv, pis. vi and vii. 



