288 BULLETIN 82, UNITED STATES NATIONAL MUSEUM VOLUME 1 



The distal edges of the radials are even with the rim of the centrodorsal in the 

 median line and are only slightly concave so that in the interradial angles the apposed 

 distal angles of two adjacent basals form only very low triangles. 



In PromachocriniLs, besides the 5 normal radials there are 5 additional similar radials 

 interpolated between them; as there are only 5 basals in the young the rosette is 5- 

 rayed, so that each of the interpolated radials is situated interradially directly over a 

 basal ray. The interradial radials do not appear hi the young until after the true 

 radials have come into contact and reached a considerable size; they then become 

 evident as narrow interradials which, growing rapidly, give rise to IBr series and gradu- 

 ally acquire all the characteristics of true radials. But it is not until the animal is 

 nearly of full size that the radial and interradial radials become quite similar (See vol. 

 1, part 2, pp. 550, 551 [Nos. 41, 42], 555; and figs. 931-937, p. 549). 



The disk is not at all or only slightly concave in the interradial areas; it is naked, 

 or bears prominent calcareous nodules of various sizes, especially near the ambulacral 

 grooves. 



Sacculi are abundant and conspicuous, and regularly distributed. 



The profile of the proximal portion of the animals when the arms are folded inward 

 is broadly rounded except in Anthometra and, to a somewhat lesser degree, in Promacho- 

 crinus in which the IBr series and arm bases lie in almost a straight line. 



The IBr series and arm bases show relatively little variation. They are robust, 

 rounded dorsally, and well separated, with prominent or thickened and usually spinous 

 borders and a greater or lesser development of spines on the dorsal surface, except in 

 Anthometra, hi which the sides of the IBr series are almost straight and nearly in contact 

 and each of the ossicles of the IBr series and lower brachials bears a high, more or less 

 sharp, median carination with a finely spinous crest, a structure frequently seen indicated 

 on the IBr series and first two brachials of the young hi other genera, though never 

 in the fully grown adult. 



The IBri are not quite in contact basally ; their sides are very strongly convergent, 

 except in Anthometra in which they are parallel. The axillaries are usually rhombic 

 with concave sides and the distal angle often truncated and grooved hi the center, about 

 as broad as long, with a more or less developed rounded posterior process incising the 

 IBr! ; in large specimens they are usually broader than long, and in young individuals 

 much longer than broad with a long posterior process which is sometimes so well devel- 

 oped (in Promachocrinus, Florometra asperrima, etc.) as to suggest the process hi the 

 larger species of Psathyrometra. The posterior portion of the axillary usually rises into 

 a broadly rounded synarthrial tubercle. 



The arms vary in length hi adult individuals from 100 to 350 mm; hi most species 

 they are between 150 and 200 mm. long. 



The arms are fairly uniform in structure, rather broad and strongly rounded dorsally 

 except in Anthometra in which they are slightly compressed and each brachial rises to a 

 sharp median crest. Between the first and second syzygies the brachials are wedge- 

 shaped to almost oblong, usually about twice as broad as the median length but often 

 shorter, with the proximal and distal edges somewhat concave; they then become more 

 obliquely wedge-shaped to triangular, usually somewhat broader than long, more rarely 

 much broader than long or as long as broad, distally becoming less obliquely wedge- 

 shaped again and gradually increasing in length. The distal ends of the brachials 

 are always prominent, rarely smooth but usually sharply serrate or spinous, sometimes 



