PTERYGOTUS GIGAS. 



81 



Thoracic Plate (formerly called the Epistoma, pi. viii, fig. 2, Mem. Geol. Surv., Mon. I). 

 — The proportions of this plate and its sculpture are very like those of i\ anglicus, and 

 specimens figured indicate this part to be quite as large as in that species. The 

 plicae on the upper or front portion are crowded, and but slightly curved ; those further 

 back are semicircular or even semi-oval, while those near the apex of the side lobes 

 are narrower and pointed as in the cognate species. Similar but smaller plicae occur 

 down the centre lobe, which does not appear to have prominent elongate scales, nor is it 

 convex as it is in P. anglicus. Its base is broad and spear-shaped. 



Fig. 17. — Head of Pterygotus gigas, Salter. From the Downton Sandstone of Kington, Herefordshire ; 



in the Cabinet of R. Banks, Esq. 



Body-rings. — The anterior body-rings (Mem. Geol. Surv., Mon. I, pi. viii, fig. 3), 

 bear the squamae only on their front half, and these are less curved and less crowded 

 than in the corresponding segments of P. anglicus. The edge of the plicae is thickened. 

 Fi^s. 4 and 5 (op. cit.) must represent large segments from a portion of the body further 

 back than fif. 3, for the squamae cover the whole lower surface of the segment (fig. 5), 

 and the greater part of the upper side (fig. 4). They are greatly more convex than in the 

 Scotch species, the posterior ones especially being parabolic or even pointed in form, 

 frequently three tenths of an inch long and equally broad. Fig. 5 (op. cit.) shows the 

 closely squamate lateral edges of the segment, which are convex and rounded in the forward 

 portion and sharply keeled behind. The front margin in both of these segments is con- 



