DESCRIPTION OF THE FOSSIL REMAINS. 71 



The pectoral fin is very much larger than the ventral, attaining to a 

 quarter of the length of tlie body. A solid axis, covered with small scales, 

 and tapering to a fine point at its extremity, traverses the whole length of 

 the fin, and has the comparatively short fin rays attached to its anterior and 

 posterior margin, as in Phaneropleuron. 



The teeth of Holoptychius are, for the most part, short, straight, slender, 

 and conical ; but the anterior part of the mandible of a specimen in the 

 Museum of Practical Geology, exhibits a long, slender, and somewhat recurved 

 tusk, more than twice as long as any of the other teeth. The surface of this 

 tooth presents a number of fine sharp longitudinal ridges. An outline of the 

 left ramus of this mandible, of the natural size, and a magnified view of the 

 tusk, are given in the woodcut. It will be observed that these teeth are 

 totally different from those of Rhizodus. 



Diplopterus Dalghisiams. — Agassiz. Pl. I. Fig. 4. 



The specimens of this genus have now been found in the several suites of 

 rocks, from the lower Old Ked to the upper Yellow Sandstone of Dura Den, 

 and likewise in the coal-measures. Like the Holoptychius^ it is therefore of 

 vast geologic range, commencing and terminating with that strong bright 

 scaled family in the same early series of deposits. The specimens obtained 

 in Dura Den consist only of two portions, a head, whicb is quite perfect, and 

 the posterior part of the body. These were forwarded by me to M. Agassiz 

 in 1837, and returned labelled as " Diplopterus, new species." 



The individuals of this genus are represented as being of great bulk, attaining 

 a length of several feet, slender, and tapering in the body, and, from the size 

 and position of the fins, powerful swimmers. The head is comparatively large 

 and well proportioned in its divisional parts. The eyes are placed in the 

 middle of the head, close on the median line, which are large, and surrounded 

 by strong osseous projections ; and the frontlets, which form the top of the 

 cranium, are flat, narrowing between the eyes, and dilating before and behind. 

 The snout is rounded ; and the teeth are placed in a simple dental cavity, are 

 destitute of folded detentition, and greatly resemble the teeth of the Poly- 

 pterus. The branchiostic rays, which are multiplied in other fishes, are, in 

 the Diplopterus, as well as in the Polypterus, supplied by two strong and 

 large triangular plates, occupying all the under part of the throat, and com- 

 prised between the two branches of the under jaws. These two moveable 

 plates are separated by the median line, and permitted the throat to dilate, 

 by imparting to it a great degree of solidity. The Dura Den fossil has this 

 arrangement of these thoracic plates entire and most beautifully preserved, 

 showing one by one their position and relations to eacb other. 



The scales of the Diplopterus, like all the ganoid fishes of the period, are 

 extremely marked in their characters, and must have subserved important 

 purposes in the functions of marine life. These organs are comparatively 

 large, massive, and compact ; in form they are rhomboidal, hooked or en- 

 grained by their oblique edges, and show a fine granulation caused by a 

 number of small holes opening on the surface. These probably are holes of 



