6 



BRITISH FOSSILS. 



in form and sculpture, widely different from those of the latter 

 genus. 



That Platygncdhus (Agassiz), if we restrict the name to the fish 

 whose caudal extremity is figured by Agassiz ("Vieux Gres Rouge,"* 

 Tab. 25), is very closely allied to Holoptycliius cannot be doubted ; 

 indeed, the only serious question regarding it, in the absence of 

 further materials for its reconstruction, seems to be, whether it 

 should really form a separate genus ; so that I may pass on to 

 another generic type, Glyptole'pis (Agassiz). 



This genus is briefly mentioned in the Kecherches sur les Poissons 

 Fossiles," ii. p. 179, but Agassiz first described and figured species of 

 it in the " Monographie des Poissons du Vieux Gres Rouge," p. 62, 

 where Glyptolepis heads the family of the " C^lacanthes," and is said 

 to comprise fishes of moderate size, with pyriform bodies, and with 

 heads which are small, short, flattened, and have an almost semi- 

 circular contour. The rami of the mandible are stated to be beset 

 throughout their length with a single series of small, equal, conical 

 teeth, which seem to approach those of Dendrodus and Holopty- 

 chius in structure, and to have a dendritic pulp cavity surrounded 

 by folded dentinal walls. Glyptolepis microlepidotus, however, 

 is said (p. 65) to possess large teeth alternating with small ones in 

 the lower jaw. The upper jaw projected beyond the lower a little, 

 and liad similar teeth. The throat was provided, as "in all ancient 

 " Coelacanths and in Polypterus, with two mobile triangular plates, 

 " which replace the branchiostegal rays/' 



The scales were delicate, rounded, and so much imbricated that 

 the anterior one sometimes covered more than half of its successor. 

 Their upper faces were entirely smooth, and covered with a delicate 

 layer of enamel, which, apart from some concentric lines of growth, 

 exhibited no ornamentation. Their inferior faces were also smooth, 

 and formed by a very delicate layer of bone. The mass of the 

 scale was formed by an osseous and spongy substance, adorned 

 with fine rays, which radiated from the centre of the scale. These 

 rays were intersected by concentric and circular lines, so that a 

 scale, whose smooth layer is worn av/ay, presents a number of small 

 elongated cells, disposed in circular series, almost like the seats of an 

 amphitheatre. 



In the " Additions et Corrections," (1. c, p. 1 40), Agassiz adds a 

 description of the scales of Glyptolepis elegans, which supplies 

 an important correction to that just given. Referring to Tab. 21a, 

 he says, " The figure 2 a represents a scale of its natural size, and 

 " fig. 2 the same magnified. The folds of the surface vjhich con- 

 " stitute the ornaments of the visible portion of the scale are more 



