IV. Trenton Limestone Edrioasters. 1914 168 



lies at the adoral end of the posterior interradius, close to the margin 

 of the right posterior groove. It is clearly seen in specimen A 

 (PI. X, Figs. 1 & 7) and B (PI. XI, Figs. 1 & 2), and indeed can 

 always be recognized whenever that portion of the test is plainly 

 exposed. Its normal appearance is that of a small, slightly curved 

 slit, approximately parallel to the right posterior ray. Its length is 

 1*1 mm. in A, 1-5 mm. in B. It widens slightly towards its adoral 

 end, deepening at the same time, and becoming darkened with matrix, 

 an appearance that indicates an oblique passage of the canal through 

 the test. Specimen B shows the margin of the slit raised in a rounded 

 rim, well defined at the adoral end and along the right side, but 

 broken up into lobes on the other side. The slit is contained in two 

 plates, and crosses the suture between them at right angles. The actual 

 canal is only in the adoral of these two plates, and it may be that 

 this plate is homologous with the interradial elements formed by the 

 fusion of the peristonual floor-plates ; but all the plates in this region 

 are so closely united that the sutures are hard to trace. The tract of 

 the adoral plate between the rim of the hydropore and the rounded 

 margin of the peristomial floor-plates bears, in specimen B, about 

 fifteen minute, closely-set pustules. Spinelets, if attached to these, 

 would have provided a filter for the inflowing water. 



Specimen E 15930 seems to show a slight branching of the slit, 

 such as occurs during ontogeny in the very earliest stages of the 

 folding that forms the madreporite of a recent Echinoid or Asteroid. 

 Such branching increases the ciliated area by which the water-current 

 is driven. The hydropore was not mentioned by E. Billings at any 

 time, hut his restored figure (1854, Eig. 10) shows that he observed 

 something in the region where we now know it to occur. 



The Under or Ad apical face has been exposed by patient 

 preparation in specimen A (PI. X, Eig. 3), but is better shown in 

 specimen C (Plate XIII). It is bounded by the distal halves of the 

 four sinistrally curved radial grooves, which, as seen from below, of 

 course appear dextral or solar. Their ends are quite definite, as 

 described above (Text-fig. 4) ; when Plate X was drawn, some 

 twelve years ago, they were still obscured by matrix, which has since 

 been removed. The concave space included by these may, as in 

 E. buchianus, be divided into three regions: the Peripheral Area, the 

 Erame, and the Central Area. 



The Peripheral Area is formed by a ring of plates serially 

 homologous with the interradials of the upper face, and, in specimen 

 A, connected with them by a single row of small plates passing 

 between the rays. These peripheral plates of the under face are 

 a little smaller than the majority of the interradials, and form, in 

 specimens A and C, a single row, occasionally doubled, bordering the 

 rays. In E 16173 (E. levis) these plates are relatively smaller than 

 in A and C, and occupy a relatively wider belt, so that on both counts 

 they are more numerous ; they differ further in being more distinctly 

 of two sizes, namely, larger plates with smaller ones surrounding or 

 intercalated between them. 



The Frame is a ring of larger and thicker plates (see the section, 

 PI. X, Eig. 9), the number of which appears to have been twelve in C, 



