38 Proceedings of the Royal Physical Society. 



margin, is entirely received in the aforesaid notch of the 

 median occipital, not extending on each side to join the 

 laterals, as in Pterichthys and Asterolepis. The laterals (l.) 

 are much broader, while the extra-laterals {e. L, B in Whit- 

 eaves's figure) are very small and narrow; but I have not 

 seen the still smaller plate which Whiteaves figures as A in 

 front of the last-named. 



The pattern of the cephalic lateral-line grooves is consider- 

 ably different from that in Asterolepis and Pterichthys. No 

 transverse commissure unites the lateral groove of each side 

 across the occipital plates, as in those genera ; but in front, 

 just at its inward 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 seen, forming an 

 angle with each other behind, whence, diverging obliquely 

 forwards and outwards, they pass also over the lateral plates 

 and terminate near the flexure of the great groove, close 

 behind the origin of its small outer branch. 



These grooves are only superficial, and have nothing to do 

 with sutures, either present or former; nevertheless, their 

 having been considered as such has, as in the case of Cocco- 

 steus, given rise to confusion in the enumeration of the plates 

 of the cranial shield. Owing to this source of fallacy, Whit- 

 eaves, like Lahusen, has numbered, in his figure of the head 

 of B. canadensis, no less than seven plates more than what 

 really exist, namely, his No. 2 in front, and on each side his 

 Nos. 2 a, 3, and 9 a, though he owns that 9a" may possibly 

 be a part of the postlateral" (external occipital). That is 

 undoubtedly the case, and in like manner 2 a and 3 are 

 portions of his prelateral (lateral) and 2 of the premedian. 

 Nos. 2 and 2 a he regards as equal to the " os terminale " in 

 Asterole2ois ; but if we turn to Pander's figure (7, tab. vi., 

 fig. 1) we shall find that similar divisions are marked off by 

 a similar groove on tlie premedian and lateral plates alto- 

 gether independently of the division between these plates 

 and the os terminale. 



