212 Proceedings of ike Boyal Physical Society. 



remove C. Miller i, Egert., and C. pusillus, M'Coy, from 

 Coccosteus altogether, placing them in Brachydeirics, the fact 

 being that they are simply synonyms of decijnens, Ag. C. 

 minor, H. Miller, once mixed up with C. j^usillus, M'Coy, 

 may possibly have to be put into a new genus on account 

 of the structure of the vertebral column, which presents an 

 appearance as if possessed of ossified centra ; ^ but I can see 

 no reason for associating this species with v. Koenen's 

 Brcichydeirus. 



The following description of the structure of the bony 

 skeleton of Coccosteus is therefore based upon an examination 

 of the common and well-known species G. decipiens, Agassiz. 



Head. — In PL XI., Fig. 2, the bones forming the cranial 

 shield are sketched, as well as the ramifications of the lateral- 

 line grooves. These bones are : — one median occipital (m. o), 

 two external occipitals {e. o.), two central plates (c), two 

 marginals (^m.),Uvo post-orbitals (pt. o.), two pre-orhitals (p. o.), 

 one posterior ethmoidal {p. e.), and one anterior ethmoidal 

 {a. e.), between which last and the premaxill?e {p. mx) the 

 nasal openings {n.) are observable. I have already (13, p. 52) 

 explained that I have applied those names without the inten- 

 tion of considering any of the bones exact equivalents of 

 bones similarly named in ordinary fishes. 



The orbit, the upper margin of which is formed by the 

 excavated outer edges of the post- and pre-orbital buckler- 

 plates, is completed below by the superior maxillary bone 

 {mx., Fig. 1), which strongly resembles in shape that of 

 typical Palseoniscidse in being broadly expanded behind, 

 where it covers the cheek, and suddenly excavated to form 

 a tapering process directed forwards under the eye to the 

 premaxilla. To the posterior margin of the maxilla is fixed 

 the jti^gal or post-maxilla, a triangular plate with posteriorly 

 directed apex, which fills up the space between the maxilla 

 and the lateral part of the body-cuirass. 



So far as I can see, the maxilla of Coccosteus decipicns does 

 not seem to have borne any teeth. But in a specimen from 

 Gamrie in the Edinburgh Museum there is distinct evidence 



^ This is apparently the species " witli a true bony vertebra" referred 

 to by Murchison in "Siluria," old ed., p. 501. 



