32 SMITHSONIAN INSTITUTION 



as these plates become shorter the interpinnulars disappear, and the pinnules 

 abut for their entire length (PI. IV, fig. i). The tertibrachs become shorter still 

 (about 0.7 mm.), until the thinner margins are knife-like, and there is no 

 interval between the brachials at their wider sides (PI. IV, fig. 4). Toward the 

 distal end of the arms the brachials become relatively longer, so that the 

 pinnules are slightly separated (PI. IV, fig. 10). The distal secundibrachs with 

 which the fixed arms begin, and the succeeding brachials, may be simply convex 

 exteriorly — the most usual case — or they may have a more or less raised trans- 

 verse girdle, sometimes beaded. Laterally, the brachials often have small, 

 buttress-like projections, which sometimes ramify into the interbrachial net- 

 work, in which the base of the arms entirely disappears, as if ending in branch- 

 ing roots. 



The arms are very strong at their bases, tapering gradually to great 

 lengths. They are observed in several specimens for a distance of 300 mm. 

 without reaching the ends ; calculating from the taper of the ultimate divisions, 

 and comparing this with the detached distal portions seen occasionally among 

 the specimens, it is clear that the arms were often at least 15 to 20 inches long. 

 They bifurcate at increasing intervals (longest at the inside of the dichotom) 

 three or four times, and frequently one of the last pair once again, thus giving 

 from 80 to 120 ultimate arms. The brachials from the tertibrachs up are 

 exceedingly short and wide throughout ; their extreme shortness is a very con- 

 spicuous feature, probably characteristic of the genus. The arms are highly 

 rounded dorsally, and have a broad, shallow, ventral furrow, which becomes 

 more deeply notched distally; the cross-section is more or less crescentic; from 

 the truncated horns of the crescent proceed relatively large, closely abutting 

 pinnulars. The form and relative proportions of these plates in the different 

 divisions of an arm are shown by the series of figures on Plate IV. 



There is so little difference between the anal interradius and the regular 

 areas that it is scarcely noticeable; one more plate in the second or third, and 

 in the fourth anal range is all that differentiates them. 



The interbrachial pavement. — The most interesting morphological feature 

 of this species is the peculiar interbrachial network, or pavement, connecting 

 the lower portions of the arms, upon the different aspects of which Messrs. 

 Waagen and Jahn founded three of their varieties under the name excavatus. 

 They have described and figured its various forms in great detail (op. cit, 

 pp. 63-88, pis. 43-55). These forms are all found to be substantially repeated 

 in the Cape Girardeau specimens, among which the interbrachial areas are either 

 (1) smooth (PI. Ill, fig. 2); (2) studded with more or less stelliform plates 

 (PI. Ill, fig. 3) ; (3) reticulate, with round or rhombic pits (PI. Ill, fig. 4) ; (4) 

 transversely banded with rounded plates in parallel, crescentic, rope-like rows, 

 arching downward as if suspended from the arms (PI. II, and PI. Ill, fig. 4). 



