24 OLD RED SANDSTONE FISHES. 



well as a series of similar furrows arising from the lateral margins (PL II, fig. 12). The 

 striations on the external surface follow the outline of the scute, as in 8c. Lloydii. They 

 are, on the average, ji^th of an inch apart near the centre of the scute, and leave a flat, 

 smooth-surfaced, crenated ridge between them. The striations do not converge poste- 

 riorly, since the outline of this part of the shield is not subacute, as in Sc. Lloydii, 

 but nearly horizontal ; hence the striations following the outline do not meet at a 

 small angle, but terminate at the posterior notch. 



Distribution. — This species occurs frequently by itself, and is often absent where 

 other species of Devonian Heterostraci abound. It is met with in Worcestershire (Hall's 

 Barn, Aightington), and in Herefordshire frequently. At Whitbach it occurs with 

 Pteraspis Crouchii. 



3. ScAPHASPis TRUNCATUS {Huxley and Salter). PL II, figs. 1 — 3. 



Syn. Pteraspis truncatus, Huxley and Salter. Quart. Journ. Geo!. Soc, vol. xii, p. 100, 

 pi. ii, fig. 1, 1856. 



Derivation. — Truncatus (cut off), from the abrupt termination of the front part of the 

 scute. ' 



Characters. — Scutum elongatum, ovatum, convexum, antice truncatura, postice attenu- 

 atum et brevispinosum ; superficie externa rugis undosis longitudinalibus, tenuissime 

 interstriatis ornata. 



Stratiyraphical Position. — Downton Sandstone, and Ludlow Bone-bed. 



History. — This species was discovered by Mr. Banks, of Kington, and described 

 by Messrs. Huxley and Salter in 1855. 



General Remarks. — This and the following species of Scaphaspis form a pair much in 

 the same way as the two Devonian species Sc. Lloydii and Sc. rectus do. Though 

 having the typical form of shield, the Silurian Scaphaspides difier from the Devonian 

 very obviously in their surface-striations, and in the presence of a median ridge, which 

 runs longitudinally at the posterior part of the shield. 



Sc. truncatus is quite a small species rarely exceeding an inch and three quarters in 

 length. A slightly elevated ridge on the median line, running from nearly the centre of 

 the scute, terminates posteriorly in a very short spine or tubercle. The general outline 

 of the scute is that of a well marked ovoid, truncated, and slightly emarginated in front, 

 and acutely terminated behind. The markings of the surface are very characteristic, 

 and, at the same time, are retained in very few specimens that I have seen, most of which 

 are indifi'erent examples of the internal cast. Prom twenty to thirty undulating ridges 



shield ; and this, periiaps, may prove a source of error in some cases in the view taken of the forms 

 of the various shields. 



