THE STRUCTURE OF TURRILEPAS PEACHI AND ITS ALLIES. 521 



these two longitudinal rows are arranged in pairs and lie on the outer surface of the 

 body, and therefore are partly hidden by the bases of the lateral plates in this inner view 

 of the series. The bases of the median plates appear to lie on a level with the anterior 

 narrow fold of the lateral plates, over which they extend laterally for one-fourth to one- 

 third their length. The plates are of an oblique subtriangular shape with their apices 

 directed forward, and they are in contact in the middle line by their inner edges. The 

 base of each plate is broad, rounded and sinuous, being somewhat excavated in the 

 middle, and thus divided into an outer broadly-rounded lobe and an inner subequal one, 

 so far as can be seen. The inner basal angle is rounded and curves sharply forwards, 

 so that the pairs of plates meet medianly in a series of acute angles. Owing to the 

 strong overlapping of the successive plates in each longitudinal series the apices are not 

 clearly seen ; but judging from isolated plates and other specimens the shape of the 

 plates was broad and short, with an apical angle of about 60°. The overlapping is of the 

 same nature as that of the lateral plates, the posterior ones successively overlapping the 

 anterior ones, so that in an inner view, as in the case of the type specimen (Plate, fig. 4), 

 the apices are hidden. The surface of each plate is marked by regular equal lamellae like 

 the lateral plates, but they are concentric to the sinuous base, and there is no sharp 

 narrow median fold, but a gentle, low, rounded submedian undulation traverses the plate 

 longitudinally to the apex. The plates seem to be flattened and not markedly convex 

 or angulated, though this appearance may be due to crushing of the specimen. 



The specimens of Turrilepas from the Drummuck beds and Starfish bed of Thraive 

 Glen (Plate, figs. 1-Sa) seem to be specifically identical with those from Whitehouse 

 Bay, and may be referred without much hesitation to T. Peachi. On one slab in Mrs 

 Gray's collection there are seven or eight of the kite-shaped plates associated, but not in 

 natural serial position, and they show all the usual characters ; two or three of the 

 median plates in an imperfect condition occur along with them. 



A very well-preserved isolated median plate from Thraive Glen (Plate, fig. 2) measures 

 6 mm. in length and nearly 7 mm. in breadth. The base is divided into two broad rounded 

 lobes, of which the inner one is rather larger, and with the inner edge of the plate forms 

 a continuous rounded curve. The outer edge (hypotenuse) is nearly straight, and slopes 

 obliquely back from the apex, which has an angle of about 75°. A low submedian 

 longitudinal fold traverses the length of the plate from the apex to the basal marginal 

 sinus. The lamellae on the outer half of the plate are rather wider apart and less 

 crowded, as well as less sharply curved, than those on the inner half. 



The most interesting specimen from the Upper Bala of Girvan is the nearly complete 

 individual from the Starfish bed (Plate, fig. 1). This has all the lateral plates in position, 

 and though their surface is poorly preserved and the median plates are only here and there 

 fairly distinct, yet the whole structure and relation of the plates can be made out, and it 

 is seen to resemble in a strikingly close manner the specimen represented by Barrande * 

 as an adult example of Plumulites folliculum from Stage Dd 2 . The posterior end of our 



* Barrande, Syst. Silur. Boheme, suppl. vol. i., 1872, p. 573, pi. xx., fig. 15. 



