CEPHALASPIDtE. 23 



2. ScAPHASPis RECTUS, LanJcestcr. PI. II, figs. 5 — 8, 12, 13 (figs. 6 and 8 are drawn 



upside down) ; and PI. VII, fig. 2. 



Syn. Pteraspis Lewisii, of Collectors, not oi Agassiz. 



Derivation. — Rectus (straight), from the straight equal form of the shield. 



Characters. — Scutum cephalicum obloiigum antice nee postice paulatim attenuatum ; 

 postice decussatum et in medio indentatum, marginibus lateraHbus rectis, parallelis, 

 superficie externa striis concentricis parallelis ornata, superficie interna colliculis obliquis 

 margin alibus notata. 



Stratigraphical Position. — Lower Old Red Sandstone (Cornstones). 



History. — By some means or other the specific name Leivisii has been connected with 

 this species, although it is not the Cephalaspis Lewisii of Prof. Agassiz. The reason of 

 this is probably that geologists finding two forms of Scaphaspis, closely allied, and 

 knowing that Agassiz had distinguished two such alhed species, concluded that one of 

 those found by them must be his C. Lloydii, the other his C. Lewisii. This conclusion, 

 however, is wrong, and I have accordingly renamed this species. 



General Remarks. — This is a much smaller species than the last, and is much less 

 widely distributed, though abundant in some quarries. The largest specimens measure 

 about two inches and a half in length. 



Scaphaspis rectus is characterised by the great narrowness and convexity of the scute, 

 and by the fact that its sides are in a great measure parallel, so that it does not present 

 the broad ovoid which we have in Sc. Lloydii, but a narrow, oblong, semicylindrical body. 

 Anteriorly, the margins of the shield converge, not, however, in a sweeping curve as in 

 the last species, but somewhat obliquely, leaving a small horizontal frontal margin. A 

 more decided differentiating character (not always preserved) is the presence (in casts) of a 

 very obvious notch or excavation in the median line at the posterior margin of the shield, 

 this margin otherwise being more nearly horizontal than in Sc. Lloydii, and presenting 

 the appearance of being truncated, instead of being produced to a subacute termination. 

 These characters are quite sufficient to distinguish Sc. rectus from Sc. Lloydii, with which 

 it is likely to be confounded. I have, however, often had great difficulty in determining 

 fragmentary specimens. With regard to the markings on the inner and outer surface of 

 the test there is little to be said. Those on the inner surface differ very little from the 

 similar furrows in Sc. Lloydii, being, however, less marked. There is a slight excess of 

 growth in an anterior direction, but far less exuberant than in the allied species. It is 

 from this cause that the lateral margins^ of the shield remain parallel. The two con- 

 verging furrows arising from the frontal margin are traceable in casts of this species, as 



^ The furrows on these margins indicate ridges in the test itself. It has to be borne well in mind that 

 the internal casts, by which most Cephalaspidce are known, only partially indicate the characters of the 



