22 UINTACKmUS: ITS STRUCTUEE AND EELATIONS. 



Ill a lower free pinnule of an adult specimen the measurements 



were : — 



Proximal pinnular . . . length .65 mm. . . . width 1.75 mm. 

 Near distal extremity . . " .50 " . . . . " .25 " 



Length of pinnule ... " 20.00 " 



The form and proportions of these lower free pinnules are well shown 

 on the enlarged photograph, PI. VIL, Fig. 5. 



Further up the arm the pinnules become more slender, and have 

 less taper, A pinnular from the upper part of an arm measures .50 mm. 

 wide and .40 mm. long ; this pinnule is 14 mm. long, the length of the 

 pinnulars remaining about the same throughout. Some of the upper 

 pinnules are longer than this. The pinnulars in these are distinctly dice- 

 box-shaped ; this is more apparent towards the distal end, where they 

 become proportionally more slender (PI. V., Fig. 9). The tenuity of the 

 pinnules in the distal part of the arms is extraordinary. They look like 

 mere threads, and narrow to a point like a needle. Indeed, so extremely 

 thin and delicate are they that it is only when they are cemented by 

 pressure to some other object — such as calyx or arm plates — that they 

 are preserved, or can be seen to the extremities (PI. VIII., Figs. A 3 and A 5). 

 The pinnulars in these parts are too slender to be measured by ordinary 

 instruments, but they are at least three times as long as wide. The form and 

 proportions of these very slender pinnules are shown best of all upon the 

 enlarged photograph at PI. VII., Fig. 6, where the pinnules are stretched 

 out upon the black ventral side of a vertically crushed calyx. 



COMPOSITION OF THE BASE. 



One new fact is disclosed by these specimens, w^hich will necessitate a 

 revision of the description of the genus heretofore given, and that is in 

 regard to the structure of the apical system, or base, of the dorsal cup. 

 This is described by Mr. Bather* as consisting of, ^^ (i) the centrale, 

 or central apical plate; (ii) 5 interradially situate basals surrounding it; 

 (iii) 5 radials surrounding the basals." As to the centrale he says: ^'Its 



* Proc. Zool. Soc. London, 1895, p. 979. 



