PART II. CHAPTER XXI. 



263 



Old Red Sandstone. 



may be divided into three principal masses : 1st, red and mottled 

 marls, cornstone and sandstone ; 2d, Conglomerate, often of vast 

 thickness ; 3d, Tilestones and paving stone, highly micaceous, 

 and containing a slight admixture of carbonate of lime. (See 

 section, p. 65.) In the uppermost of these divisions, but chiefly 

 in the lowest, the remains of fish have been found, of the genus 

 named by M. Agassiz, Cephalaspis, or tuckler-headed, from the 

 extraordinary shield which covers the head, and which has often 

 been mistaken for that of a trilobite, of the division Asaphus. 

 (See Fig. 276. p. 266.) 



Fig. 274. 



Cephalaspis Lyellii, Agass. Length 6f inches. 



This figure is from a specimen now in my collection, which I procured at Glam- 

 mis, in Forfarshire ; see other figures, Agassiz, vol. ii. Tab. 1. a, & 1. 6. 



a. one of the peculiar scales with which the head is covered when perfect. 

 These scales are generally removed, as in the specimen above figured. 



b, c. scales from different parts of the body and tail. 



A gigantic species of fish of the genus Gyrolepis has also been 

 found by Dr. Fleming in the Old Red sandstone of Fifeshire.* 



* See Agassiz, Poissons Fossiles, torn. ii. p. 139. 



