THE BONE-BED AT AUST, NEAR BRISTOL. 



423 



extending along the crown, which occupies the greatest diameter of 

 the tooth. It is slightly circular towards each end, the central 

 part being almost straight. It is very thin, and appears to have 

 constituted a sharp cutting-edge. The foldings are produced at their 

 extremities into small and separated denticles, about 24 in number, 

 whose diameter is greater from back to front than laterally. Some 

 of these are broken at the tips ; and the section thus exposed, when 

 magnified, shows that a hollow tube ascended in the centre of each. 

 The sulci descending to the body of the tooth from the denticula- 

 tions are much more marked towards the lateral extremities than in 

 the median region. Towards the base the tooth becomes gradually 

 thicker ; at the same time it also converges laterally to two thirds the 

 diameter of the crown ; from this part the tooth is broken off ; but, 

 from the impression on the matrix, it appears to have terminated in 

 a broadly expanded rounded base. The tooth is attached to the 

 matrix ; and consequently the posterior surface is not exposed. The 

 whole of the upper part of the anterior surface, above the root or 

 base, is covered with a smooth polished surface of ganoine. From the 

 base of the plications or foldings forming the crown of the tooth the 

 surface extends towards the base in the form of a semicircular 

 hollow. 



This genus of Selachians was instituted by Agassiz (Poiss. Foss. 

 torn. iii. p. 99) for the accommodation of teeth obtained from the 

 Coal-measures of Staffordshire and Lancashire. Since that time 

 specimens have been found in the Limestone of Armagh, and also 

 in the Coal-measures and Limestones of Virginia, Illinois, and other 

 localities in America. Hitherto specimens of the genus have been 

 restricted to the Carboniferous group of rocks. 



The specimen now described may either have been derived from 

 the disintegration of coal-measure strata, and washed into the 

 Rhaetic beds during their deposition ; or it may have belonged to 

 a fish which lived during the period when those deposits were 

 accumulating. It is probable that the former is the correct sup- 

 position. 



The species from Aust differs materially from the type species of 

 Agassiz, 0. apicalis. The latter is possessed of only seven or eight 

 protuberances from the crown of the tooth, the centre one being 

 considerably larger and forming an apex to those on either side. 

 C. semicircular is, N . & W .,froni the Coal-measure limestone of Ohio, 

 bears a remarkably close resemblance to 0. apicalis of Agassiz, 

 and seems to be so little removed as scarcely to necessitate a sepa- 

 rate specific name. Ctenoptycliius Orclii bears some resemblance 

 to C. denticulatus, Agass. (loc. cit. p. 101), in possessing a large 

 number of serrations closely ranged along the crown of the tooth, 

 whose lower portions form a series of plications extending to the 

 body of the tooth and there disappearing ; but in G. denticulatus the 

 curvatures extend quite straight across the crown, whilst in the 

 Aust specimen they form a semicircle. The base of the tooth in 

 C. denticulatus is also much wider than in C. Ordii. 



Ctenoptychius serratus, Ord (Sedgw. and M'Coy, Brit. Pal. Fos- 



2f2 



