254 BALANIDiE. 



prominent, blunt, produced straight downwards, making a deep longi- 

 tudinal cavity for the lateral depressor muscle; in some specimens 

 this cavity is almost arched over, so as to tend to be tubular, with a 

 short ridge in the middle (PI. 6, fig. 2 a): in other specimens there 

 is no trace of this tubular structure. Terga, with the apex beaked, 

 beak triangular, dull purple ; the longitudinal furrow is so shallow as 

 hardly to exist. The basal margin slopes down on both sides, with a 

 nearly equable curvature towards the spur ; hence the spur is broad in 

 its upper part, and narrow at its obliquely truncated lower end. Inter- 

 nally, there is an elongated dark purple patch : the shallow articular 

 furrow is of quite remarkable breadth ; the articular ridge is medial, 

 and the inflected scutal margin is not wide. The internal surface of the 

 spur is formed into a ridge, which runs a little way up the valve, and is 

 sometimes partially separated from the spur itself (fig. 2 c), making 

 the basal extremity toothed or double. The crests for the depressores 

 are pretty well developed. 



IValls, moderately strong : inner lamina slightly ribbed : the denticuli 

 on the bases of the parietal longitudinal septa are sharp : I could not 

 see any transverse septa in the parietal tubes. The radii are rather 

 narrow ; their summits are remarkably jagged and very oblique ; the 

 septa are plainly denticulated on both sides, but chiefly on the lower 

 side ; each septum itself, towards the inner lamina of the radius, branches 

 and divides: the interspaces are filled up nearly solidly. The alee have 

 apparently their summits less oblique than those of the radii : their 

 sutural edges are finely crenated. The lower edge of the sheath is hollow 

 underneath. The basis is flat ; it is rather thin, and imperfectly porose ; 

 in parts it is not at all porose, in others it is traversed only by very 

 minute pores : there is nevertheless, in some parts, even where the 

 upper layer is not porose, an underlying, cancellated layer. 



Animal's body unknown. 



24. Balantjs corrugatus. PI. 6, fig. 3 a, 3d. 



Shell white, longitudinally folded ; radii narrow. Scutum 

 internally without an adductor ridge. 



Fossil, Sub-Appemiine formations; Colle in Tuscany; Mus. Greenougb. 



I have seen only two specimens of this species attached 

 to rock, collected by Mr. Greenough, at Colle, and kindly 

 given by him to me. The species comes near to the living 

 B. crenatus, also found fossil in deposits of this same age ; 

 it differs, however, distinctly from that species, in having its 

 basis permeated by pores, and, in a less degree, in the 

 sutural edges of the radii being more plainly crenated : the 



