J. W. DAVIS ON THE GEXrS PLEURACANTHTJS, AG ASS. 323 



of the genus Comjosacantlms, Xewb., to this Society *. The general 

 characters of that genus suggest a close proximity to Pleuracanthus : 

 the teiminal opening and the internal cavity, the straight, acu- 

 minate, dagger-shaped form of the spine, and the close, compact, 

 fine-grained structure of the bone closely resemble Pleuracantlius ; 

 its method of attachment to the body, and its position immediately 

 behind the head, were, in all probability, similar in the two genera ; 

 in general appearance Comi^sacantlius presents a close approxima- 

 tion to some of the Pleuracanths, its only distinguishing feature being 

 its possession of a single row of denticles along the posterior face. 



The genus Pleuracantlius appears to have been composed of fish 

 differing in many respects from any known to exist at the present 

 time. In external form they were formidable, somewhat flat-shaped 

 fish, with a large head, large broadly expanded pectoral fins, 

 gradually tapering body, and large flat abdominal or ventral fins. 

 The tail is not well known, but appears, from imperfect impressions 

 preserved in some of the German strata, to have been unilobate, 

 with the caudal fin extending along the dorsal and ventral aspects, 

 and encircling the end, somewhat after the manner of that of an 

 eel. Immediately behind the occipital region of the head extended 

 the defensive spine. The latter was probably one fourth or one 

 fifth of the entire length of the fish. The body may have been 

 covered with minute pointed granulations or shagreen ; but in the 

 majority of the specimens the skin appears to have been without 

 scales or other protection. 



The head was massive, much depressed, and nearly round in form. 

 Extending in the form of a half circle along the anterior margin, 

 the jaws constitute a prominent feature. They were armed with 

 comparatively large, sharp, three-pronged teeth, which extended, 

 row behind row, from the circumference towards the centre of the 

 large mouth, very closely approaching the character and formation 

 found in the Sharks of the present day. The jaws and various con- 

 stituents of the cranium appear to have consisted of cartilage with 

 a multitude of closely approximating ossicular Qcntres. In the 

 fossil state this conformation has a very characteristic and pretty 

 appearance ; it was happily compared by Beyrich to a species of 

 mosaic. The little bony centres are square in form with the corners 

 rounded off, and shine with a bright and lustrous black appearance. 

 Many of the specimens from the Cannel Coal of Yorkshire are im- 

 pregnated with iron, which, in the form of pyrites, has replaced the 

 cartilaginous parts, thus encircling the black glittering enamel-like 

 specks of bone with a golden setting. 



Possibly owing to the soft and easily compressed character of the 

 skeleton of the head, the orbits in the fossils are not distinguishable, 

 and the position and size of the eyes are not known. There are indi- 

 cations, however, which might lead to the inference that they were 

 widely separated," and situated about half the diameter of the head 

 from the extremity of the snout. In the specimen described by Dr. 

 Goldfuss there are two conical depressions, pointing anteriorly out- 



* Quart. Jouru. Geol. Soc. vol. xxxri. p. G2, 



z2 



