CARADOCIAN CYSTIDEA FROM GIRVAN. 



411 



ridges, like those in an ordinary pectinirhomb ; and the resemblance to that structure 

 is enhanced by the fact that the small plates of the integument adjoining the subvective 

 tract, especially on its admarginal side, tend to run in lines continuous with these 

 ridges, just as ornament-granules frequently continue the ridges of a pectinirhomb. 

 Indeed, had one examples of this species alone, one might well have been misled into 

 regarding the structure as a pectinirhomb. Such an opinion might even have been 

 confirmed by a study of squeezes representing the same structure as seen from the 

 inside (PI. IV. figs. 40, 43) ; for here one observes only a number of sharp ridges or 

 lamellae, each apparently corresponding to a rounded ridge of the exterior. Closer 

 examination of these lamellae (text-fig. 27), in squeezes from the most favourably 

 preserved portions, reveals a median groove indicating the compound structure of each 

 lamella ; and it may further be detected that at the ends of the lamella the two halves 

 diverge and meet those of adjacent lamellae. 



The interpretation of the whole structure, then, is this : on the outer surface (text- 

 fig. 26) each elliptical food-groove appears as a single ridge, owing to its narrowness 

 and to the fact that the groove itself is covered by minute cover-plates, about fifteen 

 to the millimetre, best observed by studying an enlarged photograph under a lens. 

 The groove has no floor of stereom, so that on the inner surface (text-fig. 27) each 

 groove is clearly seen, corresponding to a ridge of the outer surface. The intervals 

 between the ridges are not visible in this view, since the edges of adjacent grooves are 

 closely pressed together to form the apparent lamellae. 



§ 219. The Vent is not clearly shown in any specimen, though the whole rectal region 

 is well exposed in (x33 and G5. In the latter specimen (PI. IV. fig. 42) the lines of 

 plates converging on the vent are obvious enough, and, so far as I can make out, the 

 vent did not lie at the apex of the angle formed by marginals 11 and 12 (or 1), but 

 was defi.ected towards marginal 10 at the side of the leg (text-fig. 25). 



§ 220. Stem. — The only specimens in which this is seen are G2 (PL IV. fig. 40), 

 which retains only the proximal region, and Gl (PI. IV. fig. 45), which preserves also 

 the median and some 7 mm. of the distal (§ 216). These latter regions are obscure, 

 which is unfortunate, since they seem to show signs of original dimeres. The dimeres 

 of the proximal region, seen in section, resemble four ridge-tiles piled one on the other, 

 with the outer slopes a little longer than the inner slopes (text-fig. 28). 



§ 221. Measurements of G. curvata in millimetres : — 





■G2. 



Gl. 



G33. 



Width of theca from toe to marginal 2 

 Width of leg at same level 

 Height from stem vertically to top of frame 

 Height from stem to top of leg . 

 Stem, proximal region, length . 

 ,, „ vi^idth 



24-5 

 7-3 



8-5 

 18 + 

 4-5 

 4 



22-8 

 7-5 



11 



17-3 

 2-9 

 2-9 



15-5 

 7-2 

 9 



13-5 



