94 LEPAS FASCICULARIS. 



out ; in some specimens, the whole lower half beneath the 

 apex of the scuta is of nearly the same width throughout ; 

 in other specimens this lower part is spear-shaped. The 

 widest part of the tergum either equals in width, or is only 

 two- thirds of the width of the widest part of the carina 

 beneath its umbo. Carina (PL I, fig. ha) highly variable 

 in shape, with the part above the umbo either spear- 

 shaped and slightly concave within, or nearly flat and 

 furnished with a central external ridge ; or the upper 

 part (fig. 6 c) is of equal and extreme narrowness through- 

 out, and deeply concave within, appearing as if only the 

 central ridge had been developed. The part below the 

 umbo, (answering to the fork in the foregoing species,) 

 is about one-third of the length of the whole valve, and 

 generally twice as wide as the upper part, but in the 

 variety with the upper part of the carina equally narrow 

 throughout, the lower part is thrice as wide as the upper ; 

 the disc, or lower part, is generally slightly concave 

 within, exteriorly either with or without a central ridge ; 

 basal margin rounded ; lateral margin more or less 

 curved, according to the form of the upper part. The 

 disc is not more deeply imbedded in membrane than is 

 the upper part of the valve. The heel or umbo is either 

 angular and prominent, or rounded. In very young 

 specimens the carina is simply bowed, instead of being 

 rectangularly bent. 



Peduncle, — short, narrow, being abruptly inflected all 

 round under the basal edges of the capitulum ; lower part 

 of very variable shape, being often suddenly contracted 

 into a mere thread (fig. 6b), which sometimes widens 

 again at the extreme end. The external membrane is 

 very thin, and is penetrated by the usual fine tubuli 

 leading to the corium; its surface is wrinkled and desti- 

 tute of spines, or with extremely few. The peduncle is 

 often completely surrounded by a yellowish ball, (of 

 which 1 have seen specimens from the coast of England, 

 and from off Borneo,) sometimes half as wide as the 

 capitulum, composed of very tender, vesicular, structure- 



