THE OLD RED SANDSTONE. 73 
the idea of a very singular little fish. The ventral fins front 
~ the space which occurs between the two dorsals, and the anal 
fin the space which intervenes between the posterior dorsal 
fin and the tail. The length of the Osteolepis, in my larger 
specimens, somewhat exceeds a foot; in the smaller, it falls 
short of six inches. ‘There exist at least three species of this 
ichthyolite, distinguished chiefly, in two of the instances, by 
the smaller and larger size of their scales, compared with the 
bulk of their bodies, and by punctulated markings on the en- 
amel in the case of the third. This last, however, is no spe- 
cific difference, but common to the entire genus, and to 
several other genera besides. The names are, Osteolepis 
macrolepidotus, O. microlepidotus, and O. arenatus.* 
Next to the Osteolepis we may place the Dipterus, or 
double-wing, of the Lower Old Red Sandstone, an ichthyolite 
first introduced to the knowledge of geologists by Mr. Murch- 
ison, who, with his friend, Mr. Sedgwick, figured and de- 
scribed it in a masterly paper on the older sedimentary for- 
mations of the north of Scotland, which appeared in the 
Transactions of the Geological Society of London for 1828. 
The name, derived from its two dorsals, would suit equally 
well, like that of the Osteolepis, many of its more recently 
discovered contemporaries. From the latter ichthyolite it dif- 
fered chiefly in the position of its fins, which were opposite, 
not alternate; the double dorsals exactly fronting the anal 
and ventral fins. (See Plate V., fig. 1.) The Diéplopterus, 
a nearly resembling ichthyolite of the same formation, also 
owes its name to the order and arrangement of its fins, 
* To these there have since been added Osteolepis major, O. inter- 
medius, and O. nanus; the two latter, however, Agassiz regards as 
doubtful. 
