II Edrioaster Buchianm. 



VII. 199 



area is 13*75 mm. ; that of the posterior area is 16-5 mm. The latter 

 also tapers less rapidly towards its actinal end, where the hydropore 

 was situated. 



The greater width of the posterior interradial area (Fig. 6 and 

 PI. VIII, Fig. 1) is due to the anal opening, which lay in this area 

 about half a millimetre to the left of its middle line and at 11 mm. 

 from the actinal pole. The anal pyramid (" ovarian pyramid " of 

 Forbes) was not a regularly constructed pyramid surrounded by 

 a definite rim, but was a roughly circular area of about 6-25 mm. in 

 diameter, the plates of which appear to have risen from under the 

 edges of the surrounding interradials. The plates contained within 

 this circle seem to have been irregularly disposed in two circlets. 

 The plates of the outer circlet were not so long as those of the 

 inner. These latter were roughly triangular and met in a central 

 point, where doubtless was the anus ; the sutures indicate their 

 number as 11, although Forbes estimated them at only 5. The 

 anal pyramid can hardly be described as raised ; but above it, 

 and half surrounding it, is a semicircular depression with three 

 concentric wrinkles plainly marked on its left side. Immediately 

 on the right of the anus, the test is not depressed. In the adjacent 

 (r. post.) interradius there is a slight elevation of the theca at this 

 level, clearly shown in the side-view (PI. VIII, Fig. 2). The 

 wrinkled depression in the anal interradius may be ascribed to 

 the contraction after death of some internal structure, presumably 

 the mesentery attaching the stone-canal to the theca. The slight 

 swelling that curves round from the anus may indicate the course 

 of the rectum ; and this would imply that the gut had a dextral 

 coil. Jaekel also has expressed this view (op. cit., p. 46). 



The Under or Abactinal Surface of the theca (PI. VIII, 

 Fig. 3, and PI. IX) is partly bounded by the dextrally curved ends 

 of the radial grooves, which, as seen from below, of course appear 

 sinistral. The excavate space included by these may be divided 

 into three regions, which, proceeding from without inwards (i.e. from 

 oral to aboral), are as follows : — 



(1) Peripheral Area. — A closed ring formed by small polygonal 

 plates like the interradials, with which they are in fact serially 

 homologous. These plates are roughly arranged in rows continuing 

 the spiral twist of the grooves. They are a little smaller than the 

 majority of the interradials, and their long axes are directed more 

 or less radially to the vertical axis of the theca. 



(2) The Frame. — A closed ring of 11 thicker and larger plates, 

 with their long axes tangential to the vertical axis of the theca. 

 Their outer margins are slightly scolloped by the sutures with the 

 plates of the peripheral area ; the inner margin of each forms 

 a curve, convex towards the aboral pole. These plates seem to 

 alternate in size, a smaller plate corresponding to each interradius 

 and a larger to each radius. Two plates go to the wider posterior 

 area. Too much stress, however, must not be laid on this slight 

 appearance of regularity. These plates were ornamented with 

 small tubercles, the traces of which are seen on both the cast and 



