VI. Pyrgocystis. 



1915 7 



E. aurantium, it seems probable that the shales themselves correspond 

 rather to the Orthoceras and Glauconite limestones which lie below 

 the Echinosphsera limestone of N.W. Europe. 



Material. — The three specimens may be denoted A, B, and C. A is 

 taken as Holotype, and, having been so generously presented to the 

 British Museum by Dr. Sardeson, is there registered as E 16,232. 

 It was collected a few months before the other specimens, and 

 probably came from a more exposed position, since it is partly 

 weathered reddish-brown, whereas they are more grey. It is also 

 rather better preserved, both as a whole and in detail, and the matrix 

 has been more easily removed. B and C remain the property of 

 Dr. Sardeson. 



General Description. — A circular oral surface of general Edrio- 

 asteroid type, with (presumably five) straight broad subvective grooves, 

 surmounts a cylindrical turret built up of scale-like plates imbricating 

 from below upwards (i.e. adorally). The diameter of the turret is 

 about two-thirds of its height. The scales bore spinules on their free 

 borders ; the oral face is obscured by longer movable spines. 



Detailed Description. — The Oral Face is preserved well enough for 

 description only in A, and here it is the perfectness of preservation 

 that militates against description, since the greater part of the surface 

 is covered by spines (PI. II, Figs. 1, 2). 



The diameter of the oral face is about 8 mm. Portions of four 

 subvective grooves are visible ; these are so situated that they appear 

 to be on four adjacent rays out of a total of five ; but which four they 

 may be it is not easy to say. Two of the interradii, which are less 

 covered with spines than the rest, are clearly neither of them the 

 anal (posterior) interradius. Of the three remaining interradii, it is 

 noteworthy that in one several of the spines, which seem to be 

 a little longer, are directed towards a point at the margin of the oral 

 face; and this suggests that the spines in question surround the anal 

 opening. Taking this as a working hypothesis, for the present 

 irrefutable, we find that the completely unexposed ray is the right 

 posterior. 



The Subvective Groove of the supposed anterior ray is seen to 

 attain a width of 1-65 mm., and to be composed of a double series of 

 plates, apparently alternating on the median line, and passing from 

 that line outwards in a distal direction, so as to form an angle of 

 about 140°. At the rounded distal extremity the angle becomes 

 more acute, as the plates assume the usual fan-like arrangement. 

 There are about five of these plates within a distance of 1 mm. An 

 imaginary section across the whole structure shows the median line 

 depressed, and the sides rising in a gentle convex curve, and then 

 sinking again towards the outer edge of the ray or groove, so that 

 this is clearly differentiated from the surrounding plates. 



Of the right anterior groove, only a small portion is exposed, and 

 this shows about five plates of one side all fenced round by spines. 

 Each of these plates bears two tubercles, one on each brow of the 

 convex curve, well raised above its general surface, and each depressed 

 in its centre. It is natural to suppose that to each of these 

 4 perforate ' tubercles was articulated a spine, and this interpretation 



