764 



THE WONDERS OF GEOLOGY. Lect. VIII. 



Dendrodus. This fish is allied to the Holoptyehius, but 

 the structure of the teeth, scales, and occipital plates, are 

 peculiar. The teeth are of a conical form, slightly curved, 

 solid throughout, and finely striated longitudinally; and 

 the medullary canals and calcigerous tubes are so disposed 

 as to produce a dendritical, or arborescent appearance, in 

 transverse sections ; a character which is expressed by the 

 name, Dendrodus.* 



Dipterus, and Dijrfopterus, are two nearly related genera 

 of ganoid fishes, so named from their possessing two dorsal 

 fins, which are placed opposite the anal and ventral fins. 

 These fishes, together with the Osteolepis, another common 

 Devonian genus, have bony scales plated with enamel, and 

 finely punctated ; jaws, consisting of enamel without and 

 bone within, and beset with sharp-pointed teeth ; " closely 

 jointed plates burnished like ancient helmets, cover their 

 heads, and seem to have formed a kind of outer table to 

 skulls externally of bone and internally of cartilage : these 

 gill-covers consist each of a single piece like that of the 

 Sturgeon ; their tails were formed chiefly on the lower 

 side of their bodies ; and the rays of their fins, enamelled 

 like their plates and their scales, stand up over the connect- 

 ing membrane, like the steel or brass in that peculiar 

 armour of the middle ages, whose multitudinous pieces of 

 metal were fastened together on a ground-work of cloth, or 

 of leather. All their scales, plates, and rays, present a 

 similar style of ornanient."f 



Glyptolepis, Cheirolepis, and C heir acanthus, are other 

 characteristic fishes of this system of deposits. J Of 



* Medals of Creation ; see p. 653. PL VI. fig. 8, represents a portion 

 of a tooth highly magnified. 



f Miller's New Walks in an Old Field, p. 80. 



X Consult Mr. Millers work above quoted; and M. Agassiz's beau- 

 tiful Monograph on the fishes of the Old Red-sandstone; published by 

 Mr. Bailliere ; and Sir It. Murchiflon'e Silurian System. 



