THE STRUCTURE OF TURRILEPAS PEACHI AND ITS ALLIES. 523 



Definition. — Body elongated oval, bilaterally symmetrical ; dorsal side gently convex, 

 subcarinate longitudinally, composed of two lateral series of large plates arranged in 

 pairs and two median rows of small plates in contact in median line, and also in corre- 

 sponding pairs. Shell thin, calcareous. Lateral plates kite-shaped, slightly curved, 

 with pointed apex, but varying somewhat in shape according to their position, with 

 their longer axes making nearly a right angle with the median carina of the body in the 

 posterior part of the organism, but becoming less and less steeply inclined anteriorly till 

 they lie nearly parallel at the anterior end. Surface of lateral plates marked by narrow 

 submedian fold along their length and usually a narrower, less impressed one on their 

 anterior half, both appearing as grooves on the outer surface and as ridges on the inner 

 surface of the plates ; [the most anterior pairs may have an anterior submarginal and a 

 posterior submarginal narrow fold in addition to the submedian one, which is always the 

 strongest]. Surface of plates crossed by regular transverse equal, closely placed and ecjui- 

 distant, imbricating lamellae at right angles to main fold and meeting the anterior edge 

 at a large angle, but curved forwards sharply towards the apex near the posterior edge of 

 the plate and more closely crowded together. Apex of lateral plates more or less pointed 

 and curved forwards ; anterior edge slightly concave, posterior edge more or less convex ; 

 base broadly rounded, convex. Bases of opposite lateral plates nearly in contact. 

 Successive plates overlap from behind forwards for about half their width, but their 

 apices are free. 



Median plates arranged in a double longitudinal series, with their inner edges in 

 contact along carina of body. Shape of plates short, broad, subtriangular, with sinuated 

 rounded base, long hypotenuse, more or less arched perpendicular ; apices directed 

 forwards and overlapping considerably from behind forwards. Surface of plates marked 

 by one or two low submedian longitudinal folds from apex to base and by regular equal 

 transverse imbricating lamellae concentric to base, meeting hypotenuse at large angle 

 but curving forward sharply at inner angle concentrically to inner edge of plate and more 

 closely crowded together. Terminal plate single, subcircular, emarginate. (See Addenda.) 



Specimens of kite-shaped plates from the Sholeshook Limestone and Redhill Shales 

 of the Haverfordwest area seem indistinguishable from Girvan examples of T. Peachi, and 

 probably it is the same species which occurs in the Staurocephalus Limestone and 

 Ashgill Shales of the Lake District. The genus is also represented, perhaps by the 

 same species, in the St Martin's beds of the neighbourhood of Haverfordwest. 



With regard to T. scotica only isolated plates are known, but they belong to both 

 the lateral kite-shaped series and to the median triangular rows. No associated series 

 in position is yet known. Etheridge and Nicholson mention only Balclatchie as the 

 locality at which this species is found, but it occurs in the same beds at Ardmillan and 

 Dow Hill, and Mrs Gray has many excellent examples of it from the latter place. 

 In the specimens of the lateral plates figured by Etheridge and Nicholson (op. cit., pi. 

 xiv., figs. 22, 23, 24) the weak narrow fold between the median one and the anterior 

 margin of the plate is not so strongly marked as in T. Peachi, but in one specimen from 



