720 Davis — On the Fossil Fish Remains of the Coal Pleasures of the British Islands. 



depth is near the posterior extremity where the jaw is 0-08 m., thence it 

 becomes less towards the symphysis, near which the jaw has a depth of 0-03 m. 

 Posteriorly the extremity consists of a concave articulatory surface {a') by 

 which it was attached to the palato-pterygoid. The substance of the jaw is 

 crushed, and appears to indicate that it was not sufficiently strong to resist 

 the pressure of the superincumbent matter. At the same time the fractured 

 surfaces show that it was by no means elastic. The anterior extremity was 

 probably of a firmer or more osseous consistence than the bulk of the jaw 

 behind ; but from the symphysis backwards the lower part of the jaw had a 

 similar texture, as indicated by the compact structure of the fractured surfaces. 

 The dentary surface was also of a firmly osseous substance, but the part of the 

 mandible between the two has the appearance of having had a thin osseous 

 covering, protecting an internal mass of more or less cartilaginous matter. The 

 surface of the bone is rugose, and where fractures have exhibited the internal 

 structure its chondroid character is clearly seen, the osseous centres presenting 

 very much the appearance of a piece of oolitic limestone, except that the colour is 

 black. The dentary surface is hidden by a large number of teeth ; the latter have 

 been disj^laced and are heaped together in a confused mass (c, c). The left 

 mandible is in a great part hidden by the teeth, the anterior portion (b) is exposed 

 compressed behind the right one. The lower jaws appear to have extended beyond 

 the upper one, but probably not to the extent indicated by the anterior extremity 

 of the upper jaw (d). 



The teeth present considerable variety of forms ; they are, however, so indis- 

 criminately mixed that it is only possible to roughly estimate the position they 

 occupied in the jaws, and the difference in form due to their location. Besides 

 the typical examples hitherto regarded as Diplodus giblosus, Ag., others with 

 more slenderly elongated cusps have been recognized by Mr. A. Smith Woodward 

 as identical with the teeth he has described as Diplodus tenuis* Another form 

 presents very much the appearance of Pleuracanthus (Triodus) sessilis described 

 by Dr. Jordan ;t and the typical forms selected by Dr. Anton Fritsch:]: as 

 representing the three genera, Orthacanthus, Pleuracanthus, and Xenacanthus, may 

 all be found in the teeth from the jaws of this specimen (PL lxvi.). 



On the lower part of the slab are two series of bones which are displaced, and 

 probably represent the branchial arches. They each consist of four or five 

 osseous elements connected together and having a semicircular arrangement. The 

 bones are similar in character to those of the jaws, consisting of closely impacted 



* Catalogue of the Fossil Fishes in the British Museum, pt. i., p. 11, pi. vi., figs. 2-4. 

 t Neues Jahrb., p. 843. 1849. 



% Fauna der Gaskohle und der Kalksteine dcr Pcrmformation Bohmens, vol. ii., pt. iv., p. 99, -wood- 

 cuts, figs. 173 A, 174 c, and 175 b. 1889. 



