370 BALANID.E. 



beneath the basal margin : it blends into the articular ridge, which 

 here projects in a remarkable manner and degree (3 b, 3 e, 3g), like a 

 rounded tooth. This tooth is in part a development of the occludent 

 ledge; it varies much in shape. The line of junction between the 

 scutum and tergum is nearly straight, and nearly at right angles to 

 their longer axes. In some specimens the scuta and terga are partially 

 calcified together. 



Terga : in three sets of specimens the terga differed considerably, 

 but as in every other respect there was the closest resemblance, I do 

 not doubt that these are merely varieties. In all three, the valve is rather 

 small, irregularly sub-triangular in shape, and externally somewhat 

 convex ; in all three, there is an occludent ledge, of variable width as 

 in the scuta ; and in all three, there is an internal tooth-like projec- 

 tion, of variable form, unlike anything I have seen in any other cirri- 

 pede. In the flrst variety (PL 13, fig. 3 c, 3g), the basi-carinal corner 

 of the valve is bluntly pointed, and a slight flexure separates this por- 

 tion of the valve from the other and scutal half, which latter thus 

 exhibits some tendency to be converted into a spur: on the internal 

 surface (3g) of this spur-like portion of the valve, there is a rather 

 long, sharp tooth, which projects rectangularly inwards ; it is flattened 

 in a plane at right angles to the longer axis of the scutum and tergum 

 together : it cannot be seen from the outside. In the second variety, 

 the shape of the valve is not very different (PI. 13, fig. 3d), excepting 

 that the flexure, separating the basi-carinal corner of the valve from 

 the spur, is deeper ; but on the internal face of the spur, the tooth is 

 far broader than in the first variety, and is flattened quite differently, 

 viz., in a plane nearly parallel to the surface of the valve, and instead 

 of projecting rectangularly inwards, it depends beneath the basal edge 

 of the so-called spur, and can be seen from the outside. In the 

 third variety (fig. 3/), the whole carina! end of the valve is cut off, 

 and there can hardly be said to be any trace of a spur, yet a slight 

 furrow apparently marks the line of separation between the basi- 

 carinal portion of the valve, here become very narrow, and the broad, 

 irregular part, which would have formed the spur had such been 

 developed : on the internal surface of the latter portion of the valve, 

 there is a very small, blunt, slightly flattened tooth, projecting inwards, 

 and more resemblmg that in the first than that in the second variety. 



Affinities : under P. crenatum I will point out the diagnosis and re- 

 lationship between this and that species. 



8. Pyrgoma crenatum. PL 13, fig. 4 a, 4 b. 



Pyrgoma c kf.xatum. G. B. Sowerby. Genera of Recent and Fossil 



Shells, (No. 218, Sept. 1S23), fig. 1 to 0. 



Scut h in much elongated, with the adductor ridge descend- 

 ing below the refexed basal margin : tergum with a broad 



