56 MONOGRAPH OF DURA DEN. 



with the frontlets nearly in the middle ; the nostrils are behind. The occi- 

 pital, and a great enamelled plate on the side of the head, indicate that it 

 was covered, like the polypteriis, by a single osseous plate, under which was 

 fixed the great masticating muscle. A specimen now in my own possession 

 tihows, in addition, the edge of the lower jaw, with one or two small teeth, 

 which is extremely strong and massive, and upwards of four inches in length. 

 The head is proportionally larger too, though considerably crushed on the 

 right side ; it is invested with irregularly marked enamelled bones, which 

 appear, as on the other specimen, to be covered by a thick and variegated 

 granulation. There are no traces of the fins in either observable, except a 

 few straggling rays which present no characters of form, size, number, or 

 position. 



A very large head of the same genus I afterwards found in one of my ex- 

 cursions to Dura Den. It measures from the snout lengthways seven inches, 

 and in breadth it is eight inches. It is considerably flattened by pressure, 

 and varies from an inch to an inch and half in thickness. The snout is 

 rounded ; both upper and lower sides of the entire head are exposed in the 

 specimen ; and the glosso-hyal, the occipital, and other plates, are in their 

 places and largely developed. This splendid specimen now forms part of the 

 Old Eed collection in the British Museum, which, along with specimens of 

 pamphractus, holoptychius, phaneropleuron, glyptolseraus, and platygnathus, 

 I presented to that national depository of science. 



Platygnathus Jamesoni. — Agassiz. 



The genus Platygnathus (PL I. fig. 2) has been established by M. Agassiz 

 upon two imperfect fossil specimens, the one consisting of a jawbone, found 

 in the Orcades by Dr. Trail, and the other being the posterior part of the 

 body which were discovered in Dura Den among the first researches made in 

 that locality. The jawbone from the Orcades, besides in itself incomplete, 

 consists only of the under half of the organ, but presenting the upper side 

 with two large broken teeth. The interior of these incisors is filled, without 

 a medullar cavity, and exhibit a radiated appearance, as in the teeth of the 

 dendrodus. Their section is perfectly circular. The external edge of the 

 jawbone is likewise supplied with several ranges of small conical teeth, united 

 the one to the other, and considerably developed. The internal edge is 

 covered by a fine granulation, having the appearance of shagreen. 



The Orcadean specimen is four inches and a half in length, and more 

 than an inch in width at the posterior part. The socket of the jaw contains 

 four large compartments nearly square, and formed by osseous supports ex- 

 tending between the external and internal branches of the jaw. 



The Dura Den fossil is much finer, fuller in development, and more com- 

 plete in details. It is twelve inches in length, six in breadth to the upper edge 

 of the dorsal fin, and seems to present about a third part of the body of the 

 fish. How marvellous the revelations of science, which can combine from 

 such fragmentary relics generic alliances, and demonstrate the one, from 



