78 KELLIADiE. 



whatsoever. The anterior side, which is greatly the longer, 

 is ample, and is rounded almost symmetrically both above 

 and below, its dorsal margin being arcuated, except near 

 the beaks, and ordinarily but little declining. The ventral 

 edge is slightly contracted near the middle, but swells out 

 in front, and ascends with rather a straightish inclination 

 on the posterior side. This latter, which is excessively 

 short, is narrowed not only by the rise of the ventral mar- 

 gin, but also by the decided slope of the hinder dorsal edge, 

 which latter rarely displays much, if any, convexity, so 

 that although the posterior tip is curved, the hinder ex- 

 tremity is not distinctly and symmetrically rounded. The 

 umbones are very prominent ; the beaks acute and inflected. 

 There is neither an umbonal ridge, nor any dorsal depres- 

 sions. The front laminar tooth is large and distinct in 

 each valve, in the right it is triangular : the hinder one is 

 very obscure, and only rudimentary, being rather the den- 

 ticular termination of the supporting plate of the large carti- 

 lage, than a veritable tooth. This shell is very diminutive, 

 being but an eighth of an inch in length, and only a tenth 

 of an inch in breadth. 



" Animal sub-oval, thick for its size, with an oblique out- 

 line, the longitudinal measure being greater than the trans- 

 verse. Body and mantle of the palest yellow, liver dark- 

 green. Foot hyaline. The mantle has the margin simple, 

 and is largely open ventrally, for the passage of the foot, 

 which has a byssal groove at its heel, from which strong- 

 filaments issue, and attach it to its usual habitat, the 

 spines of Spatangus piirpureus ; all the eleven observed 

 specimens were thus attached, and could not well be re- 

 moved by force without injuring the animal or shell. They 

 were cut off by scissors, and, on being placed in a watch- 

 glass of sea-water, immediately put forth their large foot. 



