Affinities of the Genus Pleuracanthus. 351 



by minute rhomboidal plates of enamel ; in the majority, how- 

 ever, this is not preserved and does not appear to have been 

 present, the skin being naked. The head, much depressed, 

 was very broad, and terminated in a rounded snout. The 

 position of the eyes is not indicated in any of the fossils. A 

 pair of orifices, placed a little distance behind the snout, 

 appear to indicate, in the opinion of Dr. Goldfuss, that they 

 may have been connected with the nostrils. The mouth 

 extended in a semicircular form round the anterior portion 

 of the head, the lower jaw projecting somewhat beyond the 

 upper and being of very massive construction. The mouth 

 was armed with several rows of closely-set, three-pronged, 

 sharp teeth, extending one behind the other along each 

 jaw, in a similar manner to those of the Sharks and Rays of 

 existing species. These, found separately in England, were 

 described by Prof. Agassiz, under the generic name of 

 Diplodus. 



The skeleton of the fish was, for the most part, carti- 

 laginous. The vertebra? were wholly so ; but attached to 

 them were bony ribs, short and slender. Connecting the 

 spinal column and the dorsal fin were hollow (?) spinous and 

 interspinous bones, which are preserved and were similar in 

 character to the other hard parts of the skeleton, being com- 

 posed of cartilage with innumerable osseous centres, the chon- 

 droid bone of Prof. Williamson. 



Dr. Kner says it is certain that four or five gill-arches, set 

 with a few long rake-like teeth, were present, the larger ends 

 of which were surrounded by many slender gill-rays ; they 

 were attached to the horny substance of the bones supporting 

 the tongue. The connexion of the gill-supports with the 

 shoulder-girdle resembled that of the Squalida?. The 

 shoulder-girdle does not join immediately up to the bones of 

 the head, and is not united with the vertebral column, but, as 

 in the cartilaginous fishes, it is situated so far back that 

 several of the vertebrae are in front of it. Kner discovered 

 three separate bones composing the shoulder-girdle, viz. the 

 clavicle, scapula, and suprascapula ; whilst attached to these 

 is a large, broad, bony plate composed of a single piece, 

 which, in the hinder third part, is bent on its outer edge at 

 right angles in the form of a knee ; from this springs an arti- 

 culated straight ray, which extends the whole length of the 

 pectoral fin. From the outer side of the articulated ray 

 spring many fin- rays ; and on the inner side there are also 

 a number of weaker rays. Altogether they form a very 

 large and expanded pair of pectoral fins. In an example of 



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