98 CATALOGUE OF THE BLASTOIDEA. 



0. rents (PL XVIII. fig. 9). At each side of the elevated but linear ambulacrum is a 

 narrow elongated slit, which becomes still finer when it passes beyond the limits of 

 the deltoid, and is traceable for some distance along the sides of the ambulacrum. 

 This is well shown in Meek and Worthen's figure 1 , which we had copied (PL XVIII. 

 fig. 9) before receiving a specimen of the type from Mr. Wachsmuth. 



The other American species (0. stelliformis) appears, at first sight, to present a 

 somewhat anomalous form of spiracle. In all the figures of it which have hitherto 

 been published, the ambulacra are represented as quite narrow, and as separated 

 from the spiracles by what appear to be actual portions of the radial and deltoid 

 plates ; so that these clefts would not be simply the lateral portions of the radial 

 sinus which are left unfilled by the ambulacra, but would be actually excavated in the 

 substance of the calyx-plates themselves ; and in fact in our only section of this spe- 

 cies (PL XVII. fig. 12) it does almost seem as if this were actually the case. 



The spiracles are thus described by Meek and Worthen 2 : — " So-called ovarian 

 openings, commencing one on each side near the inner ends of the pseudo-ambidacral, 

 or arm-areas, and extending outward along the margin of a broad sulcus, and near 

 the edges of these areas, for about half the length of the latter as very narrow slits, 

 widest at the inner end, where they connect with the inner ends of the internal 

 compressed tubes under the areas." And they also add the following foot-note : — 

 " These slits seem, as it were, to cut off a thin slice from each of the edges of the anal 

 and interradial pieces, as well as from the margins of the deep pseudo-ambulacral 

 sinuses of the radials. These slices are thicker near the upper (inner) ends, where 

 they sometimes become callous, and apparently anchylosed, in adult specimens, to the 

 pore-pieces so as to give the pseudo-ambulacra the appearance of greater breadth 

 there than is natural." In the same work 3 the spiracles of 0. gracilis are described 

 as follows : — " Openings usually called ovarian apertures, in the form of distinct 

 elongated slits, widest at the upper end, and extending down apparently three fourths 

 the length of the pseudo-ambulacra, so very close to the margins of the latter as 

 scarcely to leave more than a very thin intervening space above and apparently none 

 below." 



From the passages just quoted and from the figures illustrating them, it would 

 thus appear that Messrs. Meek and AVorthen considered the apparent separation of 

 the spiracles and ambulacra of 0. stelliformis by portions of the radial and deltoid 

 plates to be a character of specific value distinguishing it from 0. gracilis. We were 

 at first inclined to agree with this description of Meek and Worthen's, but we have 

 since found that what appear to be portions of the calyx-plates between the spiracles 

 and the proximal ends of narrow ambulacra in 0. stelliformis (PL XV. fig. 11) arc 



1 Report Geol. Survey Illinois, 1873, vol. v. pi. viii. fig. 6. 

 - Ibid, p. 400. 3 Ibid. p. 4GS. 



