THE BONE-BED AT AT7ST, NEAR BRISTOL. 



421 



Agassiz, in the third volume of the 'Poissons Fossiles,' describes 

 three species of this genus : — S. hamatus, from the Lias of Lyme 

 Eegis, in the collection of the Earl of Enniskillen ; S. elongatus, 

 found by Dr. Mantell in Tilgate Forest ; and S. Martini, Rob., from 

 the Portland Oolites of Linksfield. These species vary greatly in form 

 and general appearance ; but in each the generic characters are well 

 developed. The cylindrical cone forming the upper tooth-like 

 portion is bent over anteriorly, the basal portion spread out in 

 aliform processes ; and a third median ridge or process is developed 

 from the anterior surface of the base. The specimen I have de- 

 scribed from Aust shares these peculiarities ; but in detail it appears 

 sufficiently distinct to form a new species. It is also from a ho- 

 rizon earlier and lower than any of those mentioned above ; and 

 though it may probably be found necessary to modify the specific 

 relationship of these objects when more is known of them, at 

 present evidence is wanting to indicate that they are not distinct 

 species. 



Sphenonchus liamatus, Agass., is remarkable for its extremely 

 arched form and the pointed termination of the tooth-like part : it is 

 an inch in length; and its base is widely expanded. S. Martini, Rob., 

 is somewhat similar to JS. hamatus, but is shorter and less curved. 

 The third species, S. elongatus, is much larger than either of the 

 others ; the cylindrical portion is expanded at first, contracting nearer 

 the extremity, and ending in a second expansion, " en sorte que sa 

 forme ressemble un peu a celle d'une bouteille qu'on aurait recour- 

 bee." SpTienonchus obtusus differs from S. hamatus in its smaller size, 

 less expanded base, and in its curvature being at a considerably 

 smaller angle : its apical termination, wide, flattened and obtusely 

 rounded, is in marked contrast to the finely pointed end of 8. hama- 

 tus. 8. obtusus has altogether a finer and less stumpy form than 

 that of S. Martini, whilst from S. elongatus it is easily distinguished 

 by its smaller size and the nearly uniform diameter of the upper 

 portion. 



I suggest the specific name obtusus to designate this species, in 

 reference to its wide and expanded apex. The specimens are from 

 the cabinet of Mr. Ord. 



Prof. Agassiz described the genus Sphenonchus as a member of the 

 family of Hybodontes, associated with Hybodus, Cladodus, andDiplodus. 

 Since the classical work of Agassiz was completed, Diplodus has been 

 proved to be the tooth of Pleuracanthus, and must consequently be 

 removed from the family of Hybodontes, and considered in con- 

 nexion with the spine as nearly associated with the recent Rays, 

 according to Agassiz ; or, as I have attempted to show in a former 

 communication, it may have some affinities with the group of the 

 Siluroids. Sphenonchus and Diplodus are referred to in the ' Pois- 

 sons Fossiles' as offering considerable difference in microscopical 

 structure from Hybodus and Cladodus, especially the former, which, 

 whilst having a dense coating of dentine, has a large internal pulp- 

 cavity, which is very different from the tooth-structure of either 



Q.J.G-.S. jSTo. 147. 2f 



