LOWER OLD RED SANDSTONE OF SCOTLAND. 343 



approximation less or more remote in tlie jaw hinge to tLe 

 ball and socket mechanism. Now, in this respect the 

 Dipterus resembled the mammalia. As shown in an inte- 

 resting specimen which I disinterred last autumn from among 

 the rocks of Thurso, the condyles, as might be judged from 

 their character, were fitted into well-defined sockets in the 

 base of the skull. The same specimen also shows, better 

 than any other I have yet seen, the various bones of which 

 the base of the head consisted. 



The under jaw of the Asterolepia, which, like that of the 

 placoids and of most mammals, consisted, as I have said, of 

 two pieces, was hinged after a difierent fashion. The jaw 

 <jf Lepidosteus osseus is fitted into two lateral sockets, in 

 each containing two antagonistic processes, on one of which 

 the jaw shuts and opens, while the other serves as a check 

 to preserve it from opening beyond a certain width. "Were 

 it to be forcibly expanded beyond the point at which the 

 checking process comes into operation, fracture would en- 

 sue. Now, as shown by a singularly interesting specimen 

 which I found last autumn near Thurso, the jaw of Asterolepis 

 was fitted into sockets furnished with antagonistic processes. 

 [Spec. 18.] The plate on which these occur presents a curious 

 subject of study to the comparative anatomist. As shown 

 by the star-like markings on its outer side, it was a dermal 

 bone ; while, as shown by the two processes in the jaw-socket, 

 and the strong ridge on its interior surface, it performed iq, 

 the osteological mechanism of the animal the part of an in- 

 ternal one. The union, or rather identity, of the dermal 

 plate with the internal bone, though rare among the verte- 

 brates now, was by no means so in the first ganoidal ages. T 

 may, however, instance, as an example among existing fishes, 

 the scapula of the Lepidosteus osseus. It is externally a 

 naked oblong scale, and internally a true bone, deeply im- 

 bedded among the cervical muscles. Two specimens on the 



