58 



M. RUTA 



bent towards the oro-anal axis and possess a blunt, rounded or 

 plectrum-shaped posterior margin. In other specimens (e.g. PL 1, 

 figs 4-5; PI. 7, figs 2, 4-5), M', and M^are vaguely rectangular or 

 sub-semielliptical. 



Anterior thecal excavation. The anterior thecal excavation 

 occupies less than one quarter of the maximum thecal width and is 

 formed mainly by the medially directed apophyses of M', and M, 

 and, in part, by the left and right adoralia (see below). The apophyses 

 (Figs 2F, 6; PI. 6, figs 1-3) are irregularly semielliptical to semicir- 

 cular in outline and gently excavated both longitudinally and 

 transversely. A shallow groove runs almost parallel to the dorsal 

 margin of the upper half of each of the two apophyses, thus delimit- 

 ing a smaller dorsal region from a slightly larger ventral region. A 

 poorly defined, low ridge runs from the mid point of the medial 

 margin of the apophysis to its centre just below the shallow groove. 

 The ridge separates a deeper, medio-dorsal subtriangular area from 

 a shallower, latero- ventral area. 



The surface of each of the two apophyses slopes slightly forward 

 near its ventral margin, but its upper half is almost vertical. Laterally, 

 the apophyses merge smoothly into the ventral half of the dorso- 

 lateral projections of M', and M^ (see description below) from which 

 they are separated by a poorly pronounced, medially concave, cres- 

 cent-shaped, blunt lateral margin. 



Adoralia. The left and right adoralia, LA and RA, occupy the 

 most anterior part of the dorsal integument and roof over the anterior 

 thecal excavation. Their preservation is invariably poor in the speci- 

 mens examined and their margins are usually difficult to discern. In 

 plan view, they are sometimes visible immediately posterior to the 

 anterior thecal excavation (Fig. 2F-G; PI. 1. figs 2-5; PI. 2, figs 2, 5; 

 PI. 7, fig. 2). They are slightly smaller than the surrounding dorsal 

 integument plates and RA is slightly larger than LA. Both plates vary 

 in shape and size in different individuals, but this variation may be 

 the result of disrupfion, at least in part. The presence of a right adoral 

 orifice (a small opening piercing or notching RA; Ubaghs, 1968) 

 cannot be detected. Despite the poor preservation, a median adoral is 

 almost certainly absent. 



Antero-laterally, each of the two adoralia is sutured with a triangu- 

 lar facet sitting on the antero-medial part of a wedge-like structure 

 originating from the medial half of the dorso-lateral projections of 

 M'j and M| (see description of these plates below). 



LA is irregularly quadrangular to subpentagonal, slightly wider 

 posteriorly than anteriorly and about half to two-thirds as large as 

 RA. RA is usually longer than wide and subrectangular to 

 subtrapezoidal in outline, generally with a broadly convex postero- 

 lateral margin and with a gently concave medial margin 

 accommodating LA. 



Both adoralia possess a gently convex to flat dorsal surface. Partial 

 collapse of these elements as a result of burial resulted in their lying 

 slightly lower with respect to the dorsal margins of the dorso-lateral 

 projections of M', and Mj. In life, LA and RA probably formed a 

 gently arcuate structure bridging the space between the dorsal, 

 antero-medial facets of M'^ and Mp thus sitting immediately above 

 the apophyses. The space comprised between the adoralia and the 



apophyses was probably spindle-shaped to irregularly elliptical in 

 life. 



Zygal. The zygal (or zygal bar) runs obliquely from the left 

 antero-lateral angle of the theca to the postero-lateral fourth of the 

 right part of the thecal frame and is formed by a robust, posterior 

 process of plate M', and by a posterior, elongate plate sutured with 

 M^ and, perhaps, Mj. The ventral surface of the zygal is flat, whereas 

 its dorsal surface is very gently convex and with a poorly pronounced 

 subcentral keel (see below). 



The length of the posterior zygal plate is about 5/7 the length of the 

 posterior process of M' ^ . The plate is at an angle of about 30° with the 

 oro-anal axis and either forms a wide, obtuse angle with the M'| 

 process or is aligned with it. Its left and right margins are almost 

 straight and diverge slightly antero-posteriorly. The average width of 

 the plate is comparable with or slightly greater than that of the ventral 

 projections of the marginalia. The posterior third of the plate is 

 vaguely rectangular and spatulate and its left margin contacts the 

 posterior end of M^ along a gently arcuate, longitudinal suture (PL 1 , 

 figs 1 , 3-5; PL 2. figs 2, 4-5; PL 3, figs 1 , 5; PL 4, fig. 1 ). The posterior 

 margin of the posterior zygal plate is poorly preserved in the material 

 observed and cannot be reconstructed with accuracy. It might have 

 contacted the antero-medial angle of M^, assuming the correct 

 identification of this element (see above). 



The process of M\ (PL 1, figs 1-5; PL 2, figs 2, 4-5; PL 3, figs 1- 

 2, 5; PL 4, figs 1, 5; PL 5, fig. 2; PL 6, figs 1-3; PL 7, figs 2-^; PL 8, 

 fig. 3) is straight or. rarely, gently bent in its posterior half and lies 

 slightly to the left of the oro-anal axis for most of its length, forming 

 an angle of about 20°-25° with such an axis. The process is wider 

 than the ventral projections of the marginalia, especially in its 

 anterior two-thirds, and is about half as wide at its posterior end as at 

 the level of the suture between M\ and M^. The lateral margins of the 

 zygal process converge slightly posteriorly. 



Dorsal integument. The supracentralia are mostly irregularly 

 polygonal and sutured with each other along straight margins. Rarely 

 they show gently curved margins. Some supracentralia, especially 

 those situated along or in proximity to the oro-anal axis, are gener- 

 ally as wide as, or slightly wider than, the dorso-lateral projections of 

 the marginalia. The supracentralia differ in shape and size, but most 

 of them are roughly hexagonal or pentagonal. Rarely is their outline 

 triangular or quadrangular. In at least one specimen (PL 3, fig. 4), the 

 ventral surface of the dorsal integument is clearly visible. 



Although no apparent distribution pattern is observed, the median 

 and admedian supracentralia are on average larger than those sutured 

 with the medial margins of the thecal frame. Generally, two 

 subhexagonal supracentalia are distinguished from the others by 

 their larger size and by the fact that they occupy a median position on 

 the posterior half of the dorsal integument. Sometimes, a left and a 

 right longitudinal row of three slightly smaller and subpentagonal 

 plates flank the two subhexagonal supracentalia. Some of the 

 supracentralia lying along the medial margins of the thecal frame are 

 much longer than wide. The supracentralia which occupy the poste- 

 rior half of the dorsal integument are usually of irregular shape and 



PLATE 3 



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

 ammonium chloride. 1, EE 3070, x 9; almost complete ventral aspect of theca with slightly disrupted zygal bar; note styloid in right lateral view and 

 poorly preserved ossicles. 2, EE 3144, x 10; incomplete ventral aspect of theca with disrupted zygal bar. 3, EE 3101, x 9; poorly preserved ventral aspect 

 of theca, with almost complete proximal aulacophore, well preserved styloid and proximal ossicles. 4, EE 3121, x 8; internal aspect of dorsal side of 

 theca, heavily disrupted along the posterior half of its right margin; note the internal, trough-like aspect of the marginalia. 5, EE 3072, x 9; almost 

 complete ventral aspect of theca; note robust posterior zygal plate. 



