428 RAMBLES OF A GEOLOGIST. 



and branchiostegous rays ; and which serves both to support 

 the branchial arches and the branchiostegous membrane. 

 Now, in the fish of the Old Red Sandstone, if we except some 

 of the Acanthodians, we find no trace of this piece of me- 

 chanism : the arched space is covered over with dermal plates 

 of bone, as a window is filled up with panes. Three plates, 

 resembling very considerably the three divisions of a pointed 

 Gothic window, furnished with a single central mullion, di- 

 vided atop into two branches, occupied the space in the ge- 

 nera Osteolepis and Diplopterus ; and two plates resembling 

 the divisions of a pointed Gothic window, whose single cen- 

 tral mullion does not branch atop, filled it up in the genera 

 Holoptychius and Glyptolepis. In the genus Asterolepis 

 this arch-shaped space was occupied, as I have said, by a sin- 

 gle plate, that represented in the wood-cut ; and the nail- 

 shaped bone rose on its internal surface along the centre, 

 the nail-head resting immediately beneath the centre of the 

 arch, and the nail-point bordering on the isthmus below, at 

 which the two shoulder-bones terminated. Now, in all the 

 specimens which I have yet examined, the form and propor- 

 tions of this plate are such that it can be very nearly inscribed 

 in a semicircle, of which the length of the nail forms the 

 radius. A nail five inches in length must have belonged to 

 a plate ten inches in its longer diameter. I have ascertained 

 further, that this longer diameter was equal to the shorter 

 diameter of the creature's frontal buckler, measured across 

 about two-thirds of its entire length from the nape ; and 

 that a transverse diameter of ten inches at this point was 

 associated in the buckler with a longitudinal diameter of 

 fourteen inches from the nape to the snout. Thus five inches 

 along the nail represent fourteen inches along the occipital 

 shield. The proportion, however, which the latter bore to 

 the entire body in this genus has still to be determined. 

 The corresponding frontal shield in the Coccosteus was equal 



