POLLICIPES. 65 



whorl from one side of the rostral end of the capitiilum. All these valves are magnified 

 twice in fig. 1, Tab. IV; and as all, except {d), belonged to the same individual, we here 

 have the rare advantage of learning their relative sizes. The largest of the lower latera has 

 been mistaken for a scutum, and has been thus figured by Mr. Sowerby ; the mistake was 

 a very natural one, to be rectified only by examining the under side of the specimen. 

 Although the scutum is, unfortunately, at present unknown, there can be scarcely 

 any doubt that it would closely resemble that of F. gkihcr, and therefore I have not hesi- 

 tated, in this instance, to break through my rule of exclusively taking the scutum as typical 

 in PoUicipes : should, hereafter, a scutum be found in the Gault like that of P. (jlaher, it 

 may, with considerable confidence, be named as belonging to this species. 



I have felt considerable doubts regarding the nomenclature of this species : P. imguis 

 is founded on one of the latera of the lower whorl, and on what apparently is a rostrum ; 

 these valves are at present, and will probably for very long remain insufficient for the 

 foundation of a species. P. lavis is founded on a tergum and carina, and therefore on 

 better grounds. The specimen immediately to be described, leads me to believe that all 

 these valves belong to the same species ; and therefore both names were open to me. 

 Mr. Sowerby, however, has given in the same volume of the ' Geological Transactions,' the 

 name of P. lavis to a carina and tergum from Blackdown, which, I cannot doubt, is distinct: 

 Professor Steenstrup has also described a new form under the same name of lavis ; 

 such being the case, it has appeared to me advisable to take the name of P. unguis. I 

 must add, that there is some considerable variation in the terga from the Gault, which 

 renders it just possible, but not probable, that there may be a second closely allied form. 

 It is very singular, considering how very frequent terga are in all collections, that I should 

 not have seen a single scutum which could, as I believe, have belonged to this form. 



Scutum unknown. Tergum (Tab. IV, fig. 1, b, c); this is a medium-sized specimen, 

 •5 or "6 of an inch in length; it is rhomboidal, nearly flat, with a straight, slight ridge, 

 running from the apex, at about one third of the entire width of the valve from the carinal 

 angle, to the sharp basal angle : the ridge is steep on its carinal side, on which side the 

 whole surface of the valve is somewhat depressed. The upper carinal margin meets the 

 occludent margin at somewhat less than a right angle. Internally, a rather small portion 

 of the apex is marked by lines of growth ; and close along the upper carinal margin there 

 is a narrow fiurrow [b), with a ridge parallel and exterior to it ; both furrow and ridge are 

 rather variable, and, no doubt, are produced by contact with the edge of the carina. The 

 occludent margin very slightly exceeds in length the scutal margin ; the valve is widest 

 nearly across the middle. A slight depression, with just perceptibly angular sides, runs 

 parallel to the occludent margin. The scutal margin is nearly straight ; Avith the exception 

 of this latter single character, scarcely any difference can be perceived between this valve 

 and that of P. glaber. 



I have seen several large terga, {d, natural size,) nearly an inch in length, from the 

 Gault, which at first appeared so difierent, that I thought them specifically distinct ; they 



