ECHIN0THR1X DESORII. 415 



Ecliinothrix Desorii 



\Astropyga Desorii Agass., 1846, C. R. Ann. Sc. Nat., VI. 

 ! Echinothrix Desorii Pet., 1853, Monatsb. Akad. Berlin. 



Ambulacra quite pointed towards abactinal region, and much narrower, 

 than in either of the other species of the genus, while the poriferous zone is 

 extremely broad above the ambitus, nearly equalling in breadth the median 

 ambulacral space. The ambulacra are also well raised above the general 

 surface of the test, the sinking of the abactinal part of the median interam- 

 bulacral space rendering them quite prominent, There are but two inner 

 rows of secondary tubercles in the median ambulacral space, considerably 

 smaller than the two exterior ones. The coronal plates are narrow, carrying 

 near the ambitus as many as five primary tubercles on each plate ; there 

 are few secondaries irregularly scattered between them, and still fewer 

 miliaries, and these extremely small and indistinct, The primaries are 

 comparatively small, with an indistinct scrobicular circle and a not very 

 prominent mammary boss. The bare part of the median abactinal portion 

 of the test extends as far as the seventh coronal plate. On the lower 

 surface the tubercles are more closely crowded together, quite uniform in 

 size, decreasing but slightly towards the actinostoine. Actinostome small, 

 deeply lobed by the actinal cuts. Actinal membrane thin, strengthened 

 by large distant limestone plates in the prolongation of the ambulacra. 

 The abactinal system is very marked, somewhat sunken ; genital plates 

 large, broad at base, elongate, with large circular genital openings. Mad- 

 reporic genital larger than the others ; ocular plates narrow, elongate, 

 separating completely the genitals and adjoining anal system for their 

 whole length, as in the other species of the genus. Anal system nearly 

 entirely covered by a close pavement of prominent plates of uniform size, 

 of irregularly polygonal shape, each carrying a small tubercle ; a few 

 similar tubercles on the anal edge of the genital plates. 



The spines of this species are comparatively slender, stout, solid, and 

 quite short, not more than half the diameter of the test and somewhat 

 bulging at centre of shaft, usually more or less banded by narrow transverse 

 bands of greenish-yellow, though frequently the spines, as in the other 

 species, are of uniform color. 



Rod Sea ; Feejee Islands ; Sandwich Islands. 



