A MONOGRAPH OF THE EXISTING CRINOIDS 187 



Diagnostic features. — The broad and conspicuous expansion of the third-fifth 

 segments of the genital pinnules at once distinguishes this species from all others in 

 the family Thalassometridae. It is a small and delicate form with the 10-22 (usually 

 15-20) arms reaching 45-50 mm. in length, and the cirri 15-20 mm. long with 32-50 

 (usually 32-35) segments. 



The great variation in the number of the arms is very deceptive, and it is often 

 difficult to believe that the slender 10-armed individuals can belong to the same species 

 as the much stouter examples with 20 arms. 



Description. — The centrodorsal is broad, truncated conical, with the cirrus sockets 

 arranged in ten columns of three or four each, the two columns of each radial area 

 being rather widely separated from each other but closely crowded against the columns 

 of the areas adjacent. In each midradial region there is a deep V-shaped space between 

 the columns of cirrus sockets which accommodates the posterior process from the IBr 

 axillary, the rounded posterior end of this lying sometimes as much as halfway between 

 the proximal rim and the dorsal pole. When the posterior process does not descend 

 so deeply the pairs of columns of cirrus sockets are separated by a very deep and broad 

 V-shaped furrow with a V-shaped cross-section. The dorsal pole is broad, fiat, rough, 

 or papillose. 



The cirri are XII, 32-35, from 15 mm. to 17 mm. long; the longest proximal 

 segments (the 5th-7th) are twice as long as broad; from the seventh or eighth onward 

 the segments bear very prominent long and slender dorsal spines; the fifth is a well- 

 marked transition segment. 



The ends of the basal rays are visible as small rounded tubercles in the interradial 

 angles. The radials are concealed. The IBr, form a very narrow border between 

 the posterior process of the IBr 2 (axillary) and the centrodorsal. The IBr 2 are broader 

 than long with all four sides deeply concave so that the lateral quarters are exceedingly 

 narrow with parallel sides which distally rise to a rather sharp angle— somewhat less 

 than a right angle — and proximally to a relatively deep and narrow rounded posterior 

 process which is longer than the anterior angle, reaching downward to between the 

 first or second cirrus sockets of the interradial columns. The IIBr series are of two 

 elements, and resemble the IBr series. IIIBr series of two ossicles are occasionally 

 present. The division series and arm bases are deep, in close lateral contact, and 

 sharply flattened against their neighbors. 



The arms are from 10 to 20 (usually between 15 and 20) in number, about 50 mm. 

 long. There is a broad roughened finely spinular band on the arms which on the 

 triangular brachials becomes lines of serrations. 



P! is from 3.5 mm. to 5.0 mm. long, rather stout basally but rapidly tapering and 

 becoming slender beyond the sixth segment, composed of 11-14 segments of which 

 the sixth and following are longer than broad. P 2 is 2.5 mm. long, much more slender 

 than Pi and evenly tapering from the base to the tip, composed of 7 or 8 segments. 

 P 3 is smaller and more slender than P 2 , becoming very slender beyond the second 

 segment, about 2. mm. long with 8 segments. On P< and the following pinnules the 

 third-fifth segments are very greatly expanded, the expansion being at a maximum 

 on the third or third and fourth and gradually tapering away on the sixth so that the 

 pinnule as a whole has a leaflike outline. The distal pinnules are 3 mm. long with 1 1 

 segments. 



