64 



M. RUTA 



Fig. 6 JuUaecarpus milnerorum gen. et sp. nov. Late Ordovician. Upper Ashgill, Rawtheyan, Upper Ktaoua Formation. Morocco. Reconstruction of plate 

 M' and of its zygal process. A, dorsal aspect. B, left lateral aspect of most anterior part. C, right lateral aspect of most anterior part. D, internal aspect of 

 most anterior part (rear end of plate slightly tilted dorsally ). 



describe in detail only the internal surface of M'| including its zygal 

 process (Fig. 6). 



In ventral view, the boundary between M'^ and its posterior 

 process corresponds to a hypothetical, oblique line connecting the 

 posterior end of the suture between M' , and M | with the posterior end 

 of the suture between M', and M',. On the internal surface of M'^, this 

 line delimits the posterior boundary of a flat, subrectangular region 

 sloping slightly dorsalward both anteriorly and medio-laterally. The 

 anterior boundary of this region is represented by a poorly pro- 

 nounced crest with a broadly concave dorsal margin. Such a crest 

 forms the dorso-medial margin of the left apophysis. It becomes less 

 pronounced and flatter laterally, where it widens to form a blunt- 

 topped, crescentic ridge separating the apophysis from the 

 dorso-lateral projection of M'^. 



The dorso-lateral projection of M'^ (PL 6, figs 1-3) has a broadly 

 trapezoidal outline and is gently convex both longitudinally and, to 

 a lesser extent, transversely. Its ventral half is almost vertical and 

 bends sharply rearward to continue into the ventral surface of the 

 plate. The dorso-lateral projection of M'^ bends abruptly rearward at 

 the level of the medial half of its dorsal margin giving rise to a stout. 



dorsal wedge-like structure, slightly thicker medially than laterally 

 and bearing a flat, tongue-shaped posterior process. The dorsal 

 surface of this process slopes slightly downward both in a rearward 

 direction and in a medio-lateral direction. The antero-medial part of 

 the wedge-like structure is truncated, triangular in outline and flat- 

 surfaced, and articulated with the left adoral plate (see above). The 

 ventral apex of this articular surface contacts the lateral end of the 

 crest-like, dorso-medial margin of the left apophysis. The postero- 

 medial surface of the tongue-shaped posterior process is beak-like in 

 outline and deeply excavated, whereas its postero-lateral surface is 

 comparatively less expanded dorso-ventrally, less arcuate in plan 

 view and convex outward. Its medial half slopes upward laterally and 

 continues into the almost horizontal lateral half. Both surfaces merge 

 ventrally into the trough-like thecal surface of the plate, which 

 continues posteriorly into the dorsal surface of the zygal process. 



The dorsal surface of the zygal process (Fig. 6; PI. 6, figs 1-3) is 

 flat or very gently convex in transverse section in its anterior quarter. 

 The posterior three-quarters of it is occupied by a straight ridge 

 which is slightly closer to the right margin of the zygal process than 

 it is to its left margin. The ridge is wider and less pronounced in its 



PLATE 6 



JuUaecarpus milnerorum gen. et sp. nov. Late Ordovician, Upper Ashgill, Rawtheyan, Upper Ktaoua Formation, Morocco. All latex casts coated with 

 ammonium chloride. 1, EE 3143; a, d, x 12; b, c, x 20; dorsal aspect of plate M',; note dorso-lateral projection and apophysis of plate, and internal ridge 

 on its dorsal surface. 2, EE 3101, x 20; internal ridge on dorsal surface of M',. 3, EE 31 17; a, x 20; b, x 10; dorsal aspect of M'^. 



