POLLICIPES. 49 



of two lines meeting each other at a wide angle, or somewhat irregular. The angle which 

 this basal margin makes with the occludent margin varies much. The occludent margin is 

 slightly arched, and is sometimes exteriorly strengthened by a ledge or rim. A prominent 

 ridge runs in several species from the apex of the valve to the baso-lateral angle; 

 and in another set of species there is a second obscurer ridge running to a nearly middle 

 point of the basal margin : in this latter set, the two ridges no doubt mark the extent to 

 which the rostrum and upper latera overlapped the scutum. Internally there is almost 

 always a deep pit for the adductor scutorum muscle : the upper part of the valve generally 

 projects freely, and is internally marked by lines of growth ; sometimes there is a furroM^ 

 along the upper part either of the occludent or the tergal margin ; in the latter case the 

 furrow seems to receive the scuto-occludent angle of the adjoining tergum, and thus locks 

 the two valves together, as in the recent P. mitella. In two species the occludent margin 

 at the rostral angle is internally produced downwards into a depending tooth or projection. 



Terga : nearly flat, rhomboidal or sub-rhomboidal ; a line formed by the converging zones 

 of growth, or a ridge, sometimes steep only on the carinal side, sometimes steep on both 

 sides, runs from the apex to the basal angle. The basal angle is sometimes truncated. 



Carina -. is either bowed inwards or is straight ; it widens from the apex down- 

 wards more rapidly than in Scalpellum ; generally a considerable upper portion projects 

 freely ; this upper portion is always much less concave than the lower part : it is sometimes 

 filled up flat, and sometimes has even a central prominent crest ; the basal margin is either 

 bluntly pointed, rounded, or truncated ; the parietes are generally more or less inflected, 

 but they are not separated by any defined ridge or angle from the roof or tectum ; the 

 lines of growth on the parietes are transverse, or generally only slightly oblique. These 

 characters will, I believe, in nearly all cases serve to distinguish the carina of a Pollicipes 

 from that of a Scalpellum. 



Sub-carina : I know of the existence of this valve only in P. concinnus, but I cannot 

 doubt that it existed in all, or nearly all, the species. I have sometimes suspected that it 

 might possibly have been absent in P. unguis and glaher, in which the carinal latera are so 

 large. 



Rostrum and sub-rostrum : as these valves occm- in P. unguis, I have little doubt that 

 they are universal ; they are apparently present in P. concinnus ; the rostrum always resem- 

 bles the carina, but is shorter and proportionally broader ; a larger proportion, also, seems 

 always to have projected freely, caused no doubt by the more abrupt flexure of this 

 end of the capitulum : this latter character is the most certain one by w^hich the rostrum 

 may be distinguished from the carina. The sub-rostrum in P. unguis resembles the rostrum, 

 but is smaller, and exteriorly is not carinated. 



Upper latera : I know these only in P. unguis and glaber, in which they consist of a 

 flat triangular plate, and in P. concinnus, in which tliey seem to be diamond-shaped. Lower 

 latera, these in P. concinnus also seem to be diamond -shaped, as in P. cornucopia; in 

 P. unguis and glaber the apices of these little valves do not project freely, and they 



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