510 BULLETIN lOO, UNITED STATES NATIONAL MUSEUM. 



termined by examining plate 4, figures 4 and 6 of the same work. 

 These figures show the structure of the plates on Brisingella coronata 

 (G. O. Sars). 



B. trachydisca is the only species of the restricted genus Brisinga 

 which I have examined. 



In the descriptions of the species of Brisinga and of related genera 

 a formula has been introduced to denote briefly the adambulacral 

 armature. This is useful for a comparison of the armatures of re- 

 lated forms or for indicating variations within the species. If a 

 transverse line is drawn through the middle of the plate, all the 

 spines aborad of the line are listed as the numerator of a fraction, 

 and all adorad to the line as the denominator. The spinelets within 

 the furrow are each denoted by ^, the smaller subambulacral spines 

 each by i, and the large subambulacral spine, nearly always single, 

 by /. Thus if there is an aboral and an adoral furrow spinlet, one 

 adoral subambulacral spine of small size, and the major subambu- 

 lacral spine is situated exactly in the middle of the plate (it is very 



often aborad of the middle) the formula would read ^j^^ -f /; or 



i + I 

 if the large spine is aborad of the middle, ^tt- I^ practice it 



is sometimes hard to decide whether a small spinelet should be classed 

 as a minor subambulacral, or as a furrow spinelet ; that is, whether it 

 should be written i or i. 



BRISINGA TRACHYDISCA Fisher. 



Plate 145 ; plate 149, fig. 5 ; plate 152, figs. 1, lo-c ; plate 156, figs. 1, la. 



Brisinga trachydisca Fisher, 1916 b, p. 31. 



Diagnosis. — Rays 13 or 12; a multicostate form with the spinelets 

 of disk in groups of 2 to 6, spaced like pseudopaxillae ; costae 40-45 

 very prominent, irregular and sinuous, without intercostal bands of 

 pedicellariae, and with relatively coarse spinelets, the costal region 

 occupying more than a third of the total length of ray but less than 

 one-half; adambulacral plates not crowded; armature with 1 aboral 

 furrow spinelet, 1 adoral actinal spine, 1^ to If the length of the 

 plate, and 1 prominent actinal spine 3 times the length of plate. 

 Kays slender, very long; breadth of disk equals 4 to 4^ times width 

 of ray at base. 



Description. — Disk moderate, with rounded margin, the abactinal 

 surface plane and not much elevated above base of rays; spinelets 

 in groups of 2 to 6 on small, spaced, convex plates resembling 

 pseudopaxillae; spinelets slightly tapered, sharp, the tip ending in 

 several minute points, the base sometimes provided with a minute 

 crossed pedicellaria, these scattered also over the intervening integu- 

 ment. 



