

the Morphology of the Blastoidea. 227 



compressed tubes under the areas ; " and they also add the 

 following footnote (p. 466) : — " These slits seem, as it were, to 

 cut off a thin slice from each of the edges of the anal and inter- 

 radial 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 callu3, 

 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." The hydrospire-clefts 

 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 illus- 

 trating them, it would thus appear that Messrs. Meek and 

 Worthen considered the apparent separation of the hydrospire- 

 clefts and ambulacra of O. stelliformis by portions of the oral 

 and radial plates to be a character of specific value, distin- 

 guishing it from 0. gracilis. Following up this idea, we 

 pointed out last year that 0. gracilis u bridges over the gap 

 between the American and the European species ; for not 

 only are the hydrospire-clefts in the latter much wider than 

 in the former, but they are also contiguous to the ambulacra 

 without the intervention of a part of the radial plate " *. 



We have since found, however, that what appear to be 

 portions of the calyx-plates between the hydrospire-clefts and 

 the proximal ends of narrow ambulacra in 0. stelliformis are 

 really the lateral portions of wide and somewhat petaloid 

 ambulacra. In well-preserved specimens they are crossed by 

 fine lines, continuous with, but less distinct than, those which 

 start from the median groove. The latter separate the inner 

 ends of the large triangular side plates, while the former sepa- 

 rate their broader outer ends and are usually entirely oblite- 

 rated. 



9 The lancet-plate is broad and nearly fills up the radial 

 sinus, i. e. the whole space between the hydrospire-clefts. Its 

 sides slope downwards rather steeply from the narrow median 

 groove j and upon them rest the side plates, the section of 

 which at the proximal ends of the ambulacra is nearly an 

 equilateral triangle. The upper side is slightly incurved ; and 

 that portion of the curve which is immediately next to the 

 food-groove is all that is usually represented as side plate in 



* Loc cit. pp. 860. 251. 



16* 



