IV. Trenton Limestone Edrioasters. 1914 119 



In the same interradius lies the anal opening (periproct), surrounded 

 by smaller interradials. 



On the concave face are seen the end of the rays, stiffening the 

 periphery. Within them are some irregular plates continuous with 

 the interradials ; and then a circular frame of rather larger and stouter 

 plates. Loosely stretched across the space within this frame was 

 a flexible integument, filled with minute plates. This integument 

 was generally protruded round the centre in roughly U-shaped lobes, 

 apparently five in all, but often irregular, at least in the fossils. 



Detailed Description. 



This is based mainly on specimen A, but measurements and 

 occasional details are also given on the evidence of the other specimens. 

 It will soon be manifest that specimens E 16173 and E 15900 represent 

 a species distinct from E. bigabyi, to which all the other specimens 

 appear to belong. For this new species the name E. levin is proposed. 



The Periphery is roughly circular, with irregular swellings and 

 indentations formed by the rays, and varying in position according 

 to the curvature of the rays. The circular form is sometimes further 

 departed from, in consequence of either natural growth or post- 

 mortem pressure, as appears from the following measurements in 

 millimetres : — 



ABC E 15900 E 16173 

 Sagittal diameter . . 36-2 43-5 ca. 37 ca. 40 36 

 Transverse diameter . . 34-5 46-4 ca. 37 ca. 40 35-6 

 Erom the periphery, the theca rises steeply at first, but soon bends 

 adorally in a low arch ; towards the adapical face it bends rapidly 

 downwards and inwards, and turns upwards as soon as the rays are 

 passed. When specimen A is placed on a flat surface the theca 

 reaches a height of 11*5 mm., exactly one-third of its transverse 

 diameter. The adapical face is excavate to about 6*5 mm., so that 

 the length of the polar axis is about 5 mm. The specimen does not 

 appear to have undergone more compression along this axis than 

 it may have been capable of effecting spontaneously during life. 

 It is, however, probable that in life the adapical integument hung 

 lower than it is found in the fossil, so that the height of the excavation 

 would have been about half the total height of the theca under 

 normal conditions. The other specimens are not so preserved as 

 to lend themselves to exact measurements in these respects ; but, 

 broadly speaking, the larger the theca the more relatively flattened 

 does it appear. This may be due to subsequent pressure in the plane 

 of bedding. Contrariwise, E 16054, which exceptionally lay oblique 

 to the bedding plane, has had its relative height exaggerated by 

 compression. E. Billings' pi. viii, fig. \a (1858) shows a section across 

 a partly crushed specimen. If this be restored to its probable 

 original shape, the measurements are approximately: thecal diameter, 

 59 mm.; thecal height, 22 mm.; height of adapical excavation, 13 mm. 

 This is one sixth larger than any actual specimen I have seen. 



Specimens that have clearly been flattened in one or another plane 

 show some displacement of the thecal plates, and this fact, taken 

 together with the solidity of all thecal elements other than the apical 



