THE STRUCTURE OF TURRILEPAS PEACHI AND ITS ALLIES. 525 



the lamellae are not curved forward near the margins, but meet them nearly at right 

 angles, and the base also is not sinuated. Probably the difference in the shape of the plate 

 and the want of curvature in the lamellae is connected with its position in the median 

 series ; or it may be the case that in T. scotica part of the axial line of the body is 

 more strongly carinated and the plates therefore more strongly folded, or they may 

 overlap in the middle line. 



A large triangular convex plate from Dow Hill (Plate, fig. 6) resembles this strongly 

 folded plate in some respects, but it is much larger than any others referable to T. scotica, 

 and differs in certain ways from the type. It measures 14 mm. long and 10 mm. wide 

 at the base ; it is irregularly triangular in shape, with the axial line somewhat curved 

 and the apex bent inwards to the shorter inner side ; the surface is strongly convex^ 

 the plate being folded sharply along a slightly curved line nearer the inner than the 

 outer border ; the inner slope is short, steep, and slightly excavated, but the outer one 

 is gently inclined, gently convex, and more than twice as wide as the inner face, and 

 has a very shallow longitudinal median depression running back from the apex to the 

 weak sinus in the base of this face. The whole base of the plate is angulated at the 

 fold, the inner part sharply curving forward at about 45° and passing into the inner 

 margin of the plate, while the outer part of the base slopes forward obliquely with a 

 sinuous outline at about 75° to the ridge, and has a broadly rounded outer angle. The 

 lamellae, which cross without interruption the whole surface of the plate, are numerous, 

 parallel and equal, but are more closely crowded on the inner face and concentric to the 

 inner margin, while on the outer face they are less closely placed, are concentric to the 

 sinuated base, and meet the outer edge at about 60°. 



Probably this plate belongs to the median series of the body, but it can only be 

 doubtfully referred to T. scotica. 



The only other representatives of Turrilepas which I have seen from the Girvan 

 district come from the Middle Llandovery of Newlands. The small imperfect isolated 

 kite-shaped plates and fragmentary median plates are too poor for description or identi- 

 fication with any described species. 



The species of Turrilepas from the Orthis argentea zone of Haverfordwest is only 

 known to me by median plates, which, however, are rather numerous in Prendergast 

 Lane and are well preserved. They are short, broad, and irregularly triangular, with an 

 apical angle of 60° to 75° ; they are as broad as long, and are divided by a broad rounded 

 or angulated fold into very unequal halves ; the inner face is steeply inclined and 

 narrow, the outer face is flattened and nearly three times as wide. The base is 

 slightly sinuated, and the lamellae, which are closely placed, meet the outer edge 

 nearly at right angles. From numerous internal casts it is seen that the interior of 

 these plates has a smooth surface. No kite-shaped plates from these beds have come 

 under my notice. 



The species of Turrilepas from the Dufton Shales of Melmerby is represented mostly 

 by small, short, broad, triangular plates of the median series, but a few imperfect kite- 



