56 



M. RUTA 



pronounced development of the peripheral flange and of the lateral 

 thecal denticulations (both of which are normally reduced in size) 

 and in the outline and relative size of the suranal plate. 



Disruption and, occasionally, slight deformation of the fossil 

 material are partially responsible for the observed variations of the 

 thecal shape. 



Marginal plates. The marginalia are divided into a group of five 

 plates on the right side and a group of five plates on the left side of the 

 theca. The rearmost left and right marginalia, M'^ and M,, are 

 scarcely visible, due to the generally poor preservation of the poste- 

 rior part of the theca (see below). 



The marginalia which occupy the same position in the left and 

 right series with respect to the aulacophore insertion are not mirror 

 images of each other. Rather, they show small but consistent differ- 

 ences in length and, to a much lesser extent, in shape. Each of the 

 marginalia consists of a wider, dorso-lateral projection and a nar- 

 rower, ventral projection. In cross-section, the dorso-lateral 

 projections narrow slightly antero-posteriorly along the thecal frame 

 and appear to be slightly wider than, and up to twice as wide as, the 

 ventral projections, depending upon the position of the plate in the 

 left or right marginal series. Conversely, the ventral projections are 

 of almost uniform width throughout the left and the right series (e.g. 

 PI. 1, figs 3-5; PI. 2, figs 1-5; PI. 3, figs 1-5; PI. 4, figs 1-2, 4-5). The 

 dorso-lateral projections (Fig. 3) are gently to strongly convex in 

 cross-section and slope ventralward and laterally, whereas the ven- 

 tral projections are always flat. The convexity of the dorso-lateral 

 projections is slightly more accentuated at the level of the antero- 

 lateral angles of the theca and decreases rapidly in antero-posterior 

 direction, so that at the level of its postero-lateral angles, the projec- 

 tions become almost flat (PI. 1, figs 1-5; PI. 2, figs 2, 4-5; PI. 5, figs 

 1-2, 4; PL 7, figs 2, 4—6). The dorso-lateral and ventral projections of 

 the marginalia meet at an acute angle forming a sharp lateral margin. 

 Such an angle varies from about 30° in the most posterior portions of 

 the thecal frame to about 70° at the level of its antero-lateral angles. 



The thecal margins show a poorly developed flange (e.g. PL 1 , figs 

 2-5; PI. 2, figs 4-5; PL 7, fig. 2) characterized by a fibrillar stereom 

 and restricted to the anterior half of the thecal frame. 



The most anterior marginal plates, M'^ and M^ contribute to the 

 anterior thecal excavation for the aulacophore insertion (described in 

 the next section). The space through which the internal cavity of the 

 aulacophore communicates with the inside of the theca is roofed 

 over by two adoral plates (see below) and delimited ventrally by the 

 apophyses of M'j and M|. Given the complicated morphology of the 

 internal (thecal) surface of M'^ and M^ and the fact that M', contrib- 

 utes to the zygal bar through its posterior process, both plates 

 (including those parts of them which are visible from the outside in 

 dorsal view, notably their dorso-lateral projections) will be de- 

 scribed in detail in conjunction with the descripdon of the internal 

 aspect of the theca. 



The asymmetries observed in the thecal frame are mainly due to 

 the unequal size of M', and M^, the length of the latter being 

 approximately 4/5 the length of the former. Both plates are strongly 

 curved medially in plan view and contribute to the lateral half of the 

 right and left antero-lateral angles of the theca. Their dorso-lateral 



projections are wider than those of more posterior marginalia. The 

 dorso-lateral projection of M, is of approximately constant width 

 throughout the length of the plate and is slightly wider than the 

 posterior half of the dorso-lateral projection of M',. The lateral 

 margins of both plates bear poorly pronounced serrations with a 

 gently sloping anterior surface and a steeply sloping posterior sur- 

 face (PL 1, figs 2-4; PL 2, figs 1, 3. 5; PL 3, figs 1-5; PL 4, figs 1-2, 

 4-5; PL 5, figs 1-3; PL 7, fig. 2). The serrations result from the 

 intersection of the lateral margins of M', and M, with faint, trans- 

 verse undulations visible on the lateral third of the dorso-lateral 

 projections of these plates. The undulations become progressively 

 shallower dorsalward before disappearing gradually. The serrations 

 decrease slighfiy in size antero-posteriorly and disappear immedi- 

 ately posterior to the sutures between M', and M', and between M, 

 and M,. The ventral projecfions of M', and M, are of almost constant 

 width, but widen slightly near their anterior ends and show character- 

 istic embayments along their medial margins, marking a sudden 

 change in their curvature (e.g. PL 2, fig. 1; PL 3, figs 1-5; PL 4, figs 

 1-2, 4-5). 



M\ and M, are shaped like elongate rods and are of approximately 

 equal shape and size. Both plates are only gently convex laterally, 

 their dorso-lateral projections show slightly concave to gently sinu- 

 ous medial margins and their width decreases slowly 

 antero-posteriorly. Their ventral projections possess broadly con- 

 cave medial margins and are slightly narrower than their dorso-lateral 

 projections. The medial margin of the ventral projecfion of M^ is 

 often divided into a straight to gently concave posterior half and a 

 moredeeply concave anterior half (e.g. PL 2, fig. 1; PL 3, figs 1-2,4; 

 PL 4, figs 2^). 



M', and M, resemble M', and M, in their elongate and rod-like 



4 4 3 3 ^ 



aspect, but while M^ is either slightly shorter than or equal in length 

 to Mj, M'j is always shorter than M',. Their dorsolateral projections 

 are shghtly narrower than those of more anterior marginalia through- 

 out most of their length, but their posterior ends are expanded 

 medially (especially in M^) to contact the suranal plate (see below). 

 The lateral margins of M'^ and M^ are very gently convex. The ventral 

 projection of M^ widens posteriorly sending a broadly triangular, 

 medial spatulate flange in contact with the rearmost part of the 

 posterior plate of the zygal bar along an antero-posteriorly elongate, 

 straight suture. The flange forms a confinuum with a similar, medi- 

 ally projected, subhorizontal and subrectangular flange of the 

 dorso-lateral projection of M^. thus interrupting posteriorly the 

 trough-like internal surface of the plate (e.g. PL 1, figs 4-5; PL 2, figs 

 2-5; PL 5, fig. 2; PL 7, fig. 2; PL 8, fig. 3). The posterior ends of both 

 M'^ and M^ are blunt, although their poor preservation does not 

 permit their accurate reconstruction. 



In some specimens, two small and irregular skeletal fragments 

 with a characteristic reticulate stereom texture flank the suranal plate 

 and are apparently loosely attached to the posterior ends of M'^ and 

 M^ from which they are distinguished by the smaller width of their 

 anterior half (e.g. PL 1, figs 4-5; PL 2, figs 2, 4; PL 7, figs 2, 4-5; PL 

 8, fig. 3). These fragments are interpreted as extremely reduced 

 plates M^ and M'^, loosely juxtaposed to the posterior half of the 

 lateral margins of the suranal plate (see below). In some individuals 

 (e.g. PL 2, fig. 4), the medial margins of M'^ and M^ seem to be slighly 



PLATE 2 



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

 ammonium chloride. 1, EE 3079, x 8; incomplete theca in ventral aspect with poorly preserved infracentralia. 2, HE 3129, x 8; almost complete theca in 

 dor.sal aspect with disrupted supracentralia. 3, EE 3129, x 8; poorly preserved theca and aulacophore in ventral aspect, with faint impression of zygal bar 

 and poorly preserved suranal plate. 4, EE 3178, x 20; posterior half of theca in dorsal aspect, with well preserved suranal plate and posterior zygal plate in 

 contact with right posterior part of thecal frame. 5, EE 3072, x 9; almost complete but posteriorly disrupted theca in dorsal aspect. 



