318 POLLICIPES MITELLA. 



ened, slightly prominent, with the inner edge covered 

 by the yellow membrane, like the exterior surface of the 

 valve. The upper part of the tergo-lateral margin over- 

 laps a little the edge of the tergum, and receives it in a 

 furrow, — the two valves being thus locked together. 

 This furrow lies in the freely-projecting, membrane- 

 covered portion, and extends up to the apex ; it is of 

 variable depth. Internally the scuta are concave, and in 

 some old specimens to a high degree. In these latter, the 

 basal margin, towards the tergo-lateral side, is strongly 

 sinuous ; the prominences are formed by the terminations 

 of the external longitudinal ridges, and correspond to 

 the interspaces between the valves of the lower whorl. 



J. 



These ridges, which are interesting, from throwing light 

 on similar ridges in some fossil species, are present, both 

 on old and young specimens, and run from the apex of 

 the valve, in a slightly curved line, to the tergo-lateral 

 half of the basal margin, where, as we have just seen, they 

 sometimes form prominences. They consist of three or 

 even four obscure, almost confluent, ridges, of which the 

 middle one is generally (but not always) the smallest : 

 together they cover the whole of that part of the scutum, 

 which is not overlapped along the basal margin by the 

 rostrum and large upper latus ; and they seem evidently 

 clue to the growth of the shell in this interspace having 

 been freer. So, again, the three or four small, confluent, 

 component ridges have the same relation to the interspaces 

 between the small latera of the lower whorl. 



Terga large, four-sided, with the internal growing 

 surface (fig. 3 ad), almost diamond-shaped; basal angle 

 blunt, rounded; exteriorly, from the apex to the basal 

 angle there is a rather broad, very slight prominence, 

 which bears the same relation to the carina and upper 

 latus, as do the compound ridges on the scuta to the 

 rostrum and upper latus. The upper part of the scutal 

 margin forms a slightly-projecting, rounded shoulder, 

 though variable in its degree of prominence, in relation 

 to the variable depth of the recipient furrow in the scuta. 



