Dr. R. II. Traquair on the Genus Dipteriis. 5 



value. Having myself dissected Ceratodus Forsteri and also 

 very carefully examined the extensive scries of specimens of 

 Dipterus contained in the Edinburgh Museum of Science and 

 Art, I may now bring forward a few facts bearing on this 

 question, as well as also point out several errors of detail into 

 which Pander seems to have slipped in his justly celebrated 

 memoir on the Cteiiododipterini. 



It may be readily seen, on examining a series of skulls of 

 Dipterus from the Thurso flagstones (see PL III. fig. 1), that 

 the chondrocraniuin was very much more extensively ossified 

 than that of Ceratodus ; in fact its side walls were entirely 

 occupied by bone apparently as far as the interorbital region. 

 Posteriorly the bony substance of the occiput shows two 

 openings, one above the other. The lower one (w.c/i), pre- 

 serving its neatly rounded contour, is for tlie entrance of the 

 anterior extremity of the notochord into the base of the skull ; 

 while the upper [f. m), always more or less distorted by 

 crushing, is the foramen magnum, for the exit of the spinal 

 cord. The bony matter surrounding these two openings may 

 be held to represent the exoccipitals ; and in front of it on each 

 side the walls of the otic region are distinctly ossified — 

 though, from the abraded condition of this part in all the 

 skulls, it is hardly possible to trace any sutural lines marking 

 off distinct osseous elements. The side walls of the cranium 

 now pass gently outwards into a projecting wing on each side, 

 this wing presenting in front a transverse margin and exter- 

 nally a prominent angle, and, allowing for the vertical flat- 

 tening to which all the specimens showing the base of the 

 skull have been subjected, it must have passed considerably 

 downwards as well as upwards. Its upper surface, displayed 

 in some specimens, and in them seen to be gently concave, 

 must have been covered by the mandibular muscle, roofed 

 over in turn by the plates of the external cranial buckler. Its 

 anterior margin shows, just within the outer angle, a facet {x) 

 for the articulation of the mandible, internal to which the 

 palato-pterygoid plate {jyt) fits closely on, overlapping also a 

 considerable portion of its under surface. This portion of the 

 skull is clearly to be considered as " quadrate;" and, though 

 it was in all probability ossified from its own centre, no very 

 distinct line of demarcation can be traced between it and the 

 osseous covering of the otic region behind ; much less is there 

 any trace of a joint. I have never found it as a detached 

 bone ; nor have I seen it wanting in any specimen showing 

 the base of the skull, save in one, evidently long exposed to 

 the action of the sea, and in which, apparently by continued 

 weathering, and not as the fossil Avas originally entombed, 



