22 OLD RED SANDSTONE FISHES. 



the form of the shield when of smaller size, the portion within the oval furrows being 

 more convex than that which is outside and nearer the margin. In addition to these 

 large concentric impressions, there are two furrows to be observed on all casts of this 

 species, also indicated by depressions of the surface of the test, which run from the 

 slightly undulated anterior margin towards a point in the median line, varying from one 

 sixth to one third of the length of the scute from that margin. These lines indicate the 

 ' rostral region,' since they mark off the anterior portion of the scute, which may be con- 

 sidered homologous with the rostrum in Pteraspis. Similarly the elliptic furrows mark 

 off the portion corresponding to the lateral coruna. In large specimens the oblique 

 ridges are of much greater length than in the smaller ones, which is owing to the fact 

 that the whole anterior portion of the shield grows more rapidly than the posterior : this 

 is most obvious from an examination of such specimens as figs. 2 and 7, in which it is 

 clearly indicated by the lines of growth that slight addition only is made to the posterior 

 part of the shield, the whole tendency of its development being in an anterior direction. 

 A small notch or inward curvature of the anterior margin, observable in some casts in 

 this species, appears to be due to the thickening of the test, which is somewhat produced 

 at this point. 



It is hardly possible to describe the arrangement of the concentric striations on the 

 surface of the test, and indeed it is not necessary, as the artist has taken great pains to 

 render them faithfully in the drawings of the specimens submitted to him. A series of 

 small pits, arranged in pairs and diverging from the centre of the scute, mark the position 

 of muciparous glands, and are seen in figs. 4 and 8. 



The form of the minute ridges between each consecutive pair of striations appears to 

 be characteristic of the species of Heterostraci. In Scaphaspis Lloydii the striae are g'^th o/ 

 an inch apart near the centre of the shield ; at the sides they appear as though com- 

 pressed, and are only ^^ of an inch apart ; the ridge left between has a^«/, glittering 

 surface, and is obscurely serrated within the groove. Fragments of this species may be 

 readily distinguished from Pteraspis rostratus, with which it is often associated, since in 

 the latter the intervening ridges are much narrower, rounded, and have a dull surface. 



Localities. — In addition to the numerous quarries in the Lower Old Red of Hereford- 

 shire, Worcestershire, and Shropshire, which have furnished abundance of specimens of 

 Scaphaspis Lloydii, Mr. Powrie has submitted to me specimens from Forfarshire 

 (obtained by the Rev. Hugh Mitchell, of Craig, near Montrose), which, though very 

 fragmentary, preserve sufficient character to warrant the conclusion that they belong to 

 this or a closely allied species. Agassiz states that he obtained this species from all 

 localities in England where Cephalaspis Lyellii occurred. 



