LEPIDOGANOIDEI. 



153 



In the genus Dipterus (fig. 63), the two dorsals, d 1, 

 d 2, are opposite the anal a, and the space thence extending 

 to the ventrals v ; the latter being a little in advance 

 of the first dorsal. The Dipterus macrolepidohis is charac- 



- j> 



Fig. 63. 

 Dipterus macrolepidotus (Devonian). 



terized by the large size of its scales. Its remains are found 

 in the old red sandstone of many localities of Scotland and 

 England. 



In the allied genus Diplopterus the dorsals are wider 

 apart, and the teeth are larger and fewer. Four species 

 have been recognised in the middle " old red " of Gamrie, 

 Orkney, and Lethenbar. Two species occur in the carboni- 

 ferous series. 



In the genus Osteolepis the first dorsal is near the middle 

 of the back. The teeth are sharp ; not any of the species 

 exceed a foot in length : they are all from the middle " old red." 



Family II. — Acanthodii. 



The species of this family are characterized by their very 

 small scales : they are heterocercal and notochordal. There 

 is a strong spine in front of each fin. The head is large ; the 

 orbits approximate ; the mouth wide, formed chiefly by the 

 maxillaries, and opening obliquely upwards, so that they 

 have somewhat the aspect of the Uranoscopi. They have many 

 branchiostegal rays. The principal genera are from the old 

 red sandstone, and are as follows : — Cheiracanthus, with a 



