28 OLD RED SANDSTONE WISHES. 



margin, the flat portions at the sides form the lateral cornua, and terminated abruptly 

 with a rounded horizontal margin, leaving the central convex portion to pass backwards 

 unembraced, for the length of about half an inch. 



A comparison of this cast with those of Cy. BanJcsii shows considerable similarity in 

 the parts of the two scutes. 



I have named it after the Rev. W. S. Symonds, of Pendock, Worcestershire, who is 

 one of the most energetic investigators of the Old Red strata of that district. 



Genus 3. — Pteraspis. 



Syn. Ptebaspis (Kner), Huxley. Quart. Journ. Geol. Soc, vol. xii, p. 100, 1856. 

 — Lankester. Brit. Assoc. Rep. Trans. Sect., p. .58, 1864. 



Derivation. — Tm^ov a wing, aairiQ a shield ; either from the sculptured surface of 

 the shield (?) or from its supposed wing-like cornua. 



Characters. — Scutum cephalicum sagittiforme ; in septem partes divisum, — rostrum 

 conicum elongatum anterius, — svinam longam posteriorem, — discum magnum centralem, 

 quadrangularem vel ovatum rostro junctum, — duas partes orhitales, rostro et disco 

 utrinque junctas, — duo cornua later alia quicque fossa tubulari perforatum, utrinque disco 

 et parti orbitali juncta; superficies externa in partibus variis diverse striis miuutis 

 parallehs ornata ; superficies interna laevis, coUiculis paucis et fossis notata. 



General Remarks. — In this genus the Heterostraci attain the most composite form of 

 scute. Seven distinct portions are marked off as well by ridges on the lower surface of 

 the shield as by furrows, and by the arrangement of the striae on the external surface. The 

 form of the shield is no longer merely oval, or a modified ovoid, but the diverging appen- 

 dages become so far developed before, behind, and laterally that the outline somewhat 

 resembles what botanists term a sagittate leaf, though this does not express the form 

 accurately. The head-shield of Pteraspis presents a recognisably piscine appearance, 

 and the orbital apertures with the long snout and projecting perforated " cornua" prevent 

 the possibility of its true character being overlooked. The shield is composed of seven 

 pieces instead of four as in Ci/athaspis : a long conical rostrum ; a large more or less oval 

 central disc, into which is fitted a large posterior spine ; an orbital piece on each side 

 attached to the rostrum and disc ; and a lateral cornu on each side attached to the disc 

 and orbital piece, from which it is scarcely distinct. In the median line, where the 

 rostrum joins the disc, is a minute, round depression, with circular striations, perhaps 

 forming an eighth, though very diminutive, piece. The distinctness of these pieces has 

 been already spoken of (pages 17 and 18). The facts bearing on the matter, as far as 

 this genus is concerned, are the following : each of the seven parts has its own series 

 of striations, arranged transversely on the rostrum, circularly around the orbital aperture 



