52 Proceedings of the Royal Physical Society. 



In PL III., Fig. 2, I have given a sketch of tlie head of 

 Coccosteus deciinens, Ag., the superficial grooves being given 

 in dotted, the sutures in continuous lines, and as regards the 

 names I have applied to tlie bony plates, I have thought it 

 best to use as few as possible which might lead the reader to 

 infer that I considered them the morphological equivalents 

 of the cranial bones of ordinary fishes. 



Posteriorly we have tlie trapezoidal median occipital plate 



(m. 0.), flanked on each side by the triangular external occipital 



(e. o). In front of these are the two central plates (c), external 



to which and forming the antero-external margin of the buckler 



are three plates, marginal (m.), post-orbital (pt. o.), and pre- 



orhital {p. o), the two latter forming the upper margin of the 



orbit. The two pre-orbitals come together in the middle line 



posteriorly only for a very short distance, in front of which 



they are separated by a narrow oval space open anteriorly, 



and in this space is lodged, first a small elliptical plate, the 



posterior ethmoidal {pt. e) and the median limb or stalk of 



the anterior ethmoidal {a. e.). This latter bone, the " pre- 



maxilla " of Huxley, is like a nail-head with a broken-off 



stalk attached, the head forming the anterior point of the 



buckler, the stalk passing back between the pre-orbitals. 



On each side of it in front are the small nasal openings (?i.), 



each being bounded externally by a small separate bone, the 



premaxilla (p. mx). The orbit is bounded below by the 



" paddle-shaped " bone {mx., Fig. 3) representing the maxilla, 



which has a sort of resemblance to that in Palavniscus, and 



to the posterior extremity of which is appended a small 



triangular plate (/), the jugal or post-maxillary. 



Turning now to Homosteiis, we shall find that Hugh 

 ^Miller's comparison of its cranial plates with those of Coc- 

 costeus is not so far amiss, of course, taking into account the 

 faults of his reading of the buckler of the latter genus. In 

 Fig. 1, I have sketched^the configuration of the specimen of 

 Homostens, referred to by John ^Tiller as having the dorsal 

 plate in situ. Much of the bony substance having splintered 

 olf from the " specimen " itself, I have had a plaster impres- 

 sion taken from the "counterpart," which consequently 

 reproduces the details of the original in a very much more 



