POLLICIPES MITELLA. 319 



Externally, parallel to the occludent margin, and close 

 below the prominent shoulder, just mentioned, there is 

 a slight and variable depression, extending up to the apex 

 of the valve. This depression is clue to the prominence, 

 variable in degree, of the tergal edge of the recipient 

 furrow in the scuta. 



Carina, triangular, strong, inwardly bowed, generally 

 with a large upper portion freely projecting ; exteriorly 

 with a narrow, sharp, central ridge or keel, which is solid, 

 the interior concavity not reaching so deep ; inner growing 

 surface (fig. 3 b\ b) deeply concave, triangular. Basal 

 margin square — that is, transverse to the longer axis of the 

 carina, or it even rises (as is best seen in the growth- 

 ridges) a little towards the exterior keel. On each side 

 of the central exterior keel, there is a narrow longitudinal 

 ridge, corresponding with the interspace between the 

 sub-carina and the next-but-one latus of the lower whorl ; 

 the latus next to the sub-carina is very small, and over- 

 lies the ridge itself. In a very large specimen, these 

 lateral longitudinal ridges formed (as they likewise did on 

 the rostrum) slight prominences on the basal margin. In 

 one specimen the carina was straight. 



Rostrum closely similar, in almost every respect, to the 

 carina, even to the exterior, lateral, longitudinal ridges, 

 and in their relation to the interspaces in the lower whorl. 

 The valve is generally not so long, but rather wider, more 

 inwardly bowed, and with the exterior solid keel less 

 prominent than in the carina. The inner growing surface 

 (fig. 3 b' d) is less acuminated at its upper end. 



Upper pair of Later a. — These are much larger than 

 the remaining valves of the lower whorl ; they are straight, 

 triangular, and much acuminated, with their apices, when 

 well preserved, extending far up, for fully three fourths of 

 the height of the scuta. They nearly equal in length the 

 carina. The growing surface (fig. 3 b\ a) is flat, triangular, 

 in well-preserved specimens forming only a third or a 

 quarter of the entire length of the valve. In the middle 

 of the basal margin there is a very slight prominence, 



