CARADOCIAN CYSTIDEA FROM (IIRVAN. 375 



8 49. I have hinted (§ 47) that one face of the theca might be concave. What is 

 rather more probable is that one face was at any rate more convex than the other, 

 especially in the lower third. In D. Sedgwicki this convexity was strengthened by 

 a folding of its plates, and in that species it seems to have been on the face directed 

 towards the observer when the appendage is to his left (PL I. figs. 5, 6). In the other 

 species, where the plates are smooth, it is less easy to form any opinion as to the relative 

 position of this convex face. The position of the face with regard to gravity is 

 discussed later (§ 593). 



§ 50. The Plates composing the theca are tessellate, although in the fossils post- 

 mortem pressure, especially near the edges, may often have produced accidental im- 

 brication. The plates are irregular polygons, varying considerably in size in each 

 individual, but also showing recognisable limits of variation for each species. Ex- 

 cluding the plates immediately adjoining the stem, the appendage, and the vent, the 

 limits for the genus are probably not less than '5 mm. and not greater than 7 mm., 

 the measurement in each case being the greatest diameter of the plate. 



§ 51. The thickness of these plates, excluding special variations, appears to be not 

 less than "25 mm., and not greater than "5 mm. 



§ 62. As a rule the plates are smooth or slightly granular, and either approximately 

 flat or slightly swollen. In D. Sedgwicki, however, a tract in the proximal third of 

 the theca, in the middle of one face and apparently (but not quite certainly) confined 

 to that face, is composed of plates with the edges folded in such a way that there is a 

 smooth rounded umbo from which a ridge leads to each angle of the plate. The imprint 

 of this is well shown in Barrande's pi. 26, f. 7. 



§ 53. It is only in D. Sedgivicki, among the available species, that the Stereom is 

 occasionally preserved. It is undoubtedly composed of calcite, which is deposited in 

 a fine homogeneous meshwork, devoid, in most plates, of definite radiating or fascicular 

 structure, so far as can be seen on the surface under a lens. The general eff"ect is fairly 

 well given by Barrande's plate 26, f 9, enlarged about 5 diameters. Barrande, it is 

 true, attributes the punctation there shown " a des molecules de schiste repandus sur 

 la surface " ; but in the British Museum specimens there are undoubted fragments of 

 reticular stereom. In D. Barrandei and D. scotica the microscopically granular 

 appearance of the imprints may very well be due to the grains of the quartzite and 

 sandstone matrix. This must not be confused with the coarser granular ornament 

 sometimes seen. 



§ 54. The wall of the theca is described by Barrande as consisting of the three 

 layers to which I have applied the terms epistereom, mesostereom, and hypostereom. 



His evidence for the existence of Epistereom, or epidermis as he calls it, consists 

 in the mamelon and ridges on certain plates, as previously described ; and he seems to 

 have believed that this was essentially a feature of all the thecal plates. Certainly the 

 latter belief was unfounded ; and as for the folding of the plates, I believe that it afi"ects 

 the whole stereom and is not a mere ornament due solely to the epistereom, for which 

 TRANS. ROY. SOC. EDIN., VOL. XLIX. PART II. (NO. 6). 50 



