POLLICIPES. 51 



2. PoLLiciPEs ooLiTicus. Tab. Ill, fig. 2. 



PoLLiciPES OOLITICUS. Buckmati. Outline of the Geology of Cheltenham, by Sir R. 

 Murchison, new edit, by James Buckmanand H, Strickland, 1845, Tab. iii, fig. 7. 



P. scutis triangiilis ; siiperjicie undulatd ; margine basalt rectangule ad marginem rectum 

 tergo-lateralem posito ; segmentum tergo-laterale a lineis rejiexis incrementi formatum deest. 

 Carina pane redd, semicglindricd, margine basali qiiadrato. 



Scuta triangular ; surface undulatory ; basal margin at right angles to the straight 

 tergo-lateral margin ; there is no tergo-lateral segment formed by upturned lines of growth. 

 Carina nearly straight, semicylindrical, with the basal margin square. 



Stonesfield Slate, Lower Oolite : Eyeford. Mus. Buckman, and Geolog. Soc. 



My materials consist of several scuta, terga, and carinae, kindly lent me by Professor 

 Buckman, and of another set (which includes the rostrum) presented by him to the 

 Geological Society of London. 



Valves : these have a smooth surface, but are undulatory in the direction of the lines of 

 growth ; at the cessation, apparently, of each zone of growth, there was a tendency to form 

 a projecting ridge or plait, as takes place in a far more marked manner in some of the 

 cretaceous species, namely, P. clegans aw^faUax. There are also excessively fine, longitu- 

 dinal striae, wdiich can be seen only when the valves are held in particular lights ; these 

 seem to have been formed by the so-called epidermis, which we know in the recent P. 

 mitella is longitudinally and finely ribbed. Scuta (fig. 2, c) but slightly convex ; triangular ; 

 basal margin straight, forming a right angle with the tergo-lateral margin, and rather less 

 than a right angle with the slightly arched occludent margin ; the tergo-lateral margin is 

 straight, and not at all protuberant : in the figure the left hand is, as usual, the occludent 

 margin; I mention this because the valve has a reversed appearance, owing to the unusually 

 small angle which the occludent makes with the basal margin. Terga (fig. 2, d) rhom- 

 boid al, slightly convex, with a rounded ridge, which is central, running from the apex 

 to the broad, rounded basal angle ; the upper carinal and occludent margins stand at 

 right angles to each other, and are short compared to the scutal and lower carinal margins ; 

 there is no trace of a depression parallel to the occludent margin. Carina (fig. 2, a, b) 

 elongated, triangular ; scarcely at all bowed inwards ; not even s^^i^-carinated ; basal margin 

 rounded, not at all protuberant. The Rostrum differs from the carina only in its greater 

 breadth compared to its length. 



Bimensions. The largest scutum is "6 long, but as there is a broken tergum about 1"1 

 long, no doubt the species attained a rather large size ; the longest carina is "7 in length. 



diagnostic characters. This species is best characterised by the straightness of the whole 

 tergo-lateral and of the basal margin of the scuta; by the ridge being central on the terga; 

 by the carina not being carinated ; and by the sinuous state of the surface of the valves, 

 intermediate between the smooth species and those with distinct ridges parallel to the zones 



