70 



BRITISH FOSSILS. 



gracilis, but the difference of the depth to the length in an individual 

 of similar length, namely four inches, would be a quarter of an inch 

 in excess. In consequence of this striking feature the Farnell 

 Diplacanthus is at once recognized by its slender form and elegant 

 proportions. The head is small, but remarkable for the large size 

 of the oral aperture. The dentigerous bones, from the posterior 

 angle of the mouth to the snout, measure three quarters of an inch. 

 They seem to have been armed with a single row of conical teeth, 

 the impressions of which are discernible, although rather indistinct. 

 In this character the genus Diplacanthus resembles Cheiracanthus 

 and Cheirolepis, and departs from Acanthodes. The orbit is placed 

 well forward, and rather high in the skull. The arrangement of 

 the opercular bones and branchiostegous rays corresponds with that 

 of the homologous parts in the genus GJieir acanthus. The pectoral 

 spine on either side is attached to a strong coracoid bone, having 

 a straight styliform shaft, and an expanded base. The spines 

 themselves are long, straight, and slender. The ventral spines are 

 about half the length of the pectoral spines, and slender in pro- 

 portion. They are situated nearer to the anal fin than to the 

 thoracic arch. The position of the first dorsal spine is the most 

 distinctive feature of the species. In all the other Diplacanths it 

 is inserted immediately behind the supraoccipital process, nearly 

 above the base of the pectoral fins, and is the strongest spine of the 

 series, whereas in this species it is situated midway between the 

 occiput and the second dorsal spine, over the interspace between 

 the pectoral and ventral fins. It is also smaller than the second 

 dorsal and anal spines. These characters may hereafter prove of 

 generic value. The second dorsal spine occupies its normal place 

 immediately opposite the anal spine. It is quite straight and much 

 longer than the first dorsal spine. The anal spine resembles it in 

 all respects except in size, in which it is rather inferior. All these 

 spines are straighter and slighter than the fin defences of the allied 

 species. The base of the caudal fin is large, the lower lobe rounded 

 posteriorly, and the fin broad. The upper lobe is deficient ; some 

 traces remain of the endoskeleton in the preservation of the superior 

 spinous processes of the anterior portion of the vertebral column, 

 which seem to have been partially if not wholly ossified. The 

 scales are minute, having a rhomboidal outline and a granular 

 surface. 



Locality. — The specimen figured was found in the Farnell beds, 

 associated with Acanthodes Mitchelli, Climatius reticulatus and 



