POLLICIPES. 63 



Northfleet, present some slight differences, in the valve being flatter, in the tergo-lateral 

 portion being more developed, and in the basal margin being straighter ; but these differ- 

 ences are so very slight that it would be exceedingly rash to consider them as specific. 



Terga (fig. 10, e, a) ; rhomboidal, flat, with a straight slight ridge running, at about one 

 third of the entire width of the valve from the carinal angle, to the sharp basal angle ; this 

 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 : the occludent margin slightly exceeds in length the scutal margin ; the valve 

 is widest nearly in its middle. A narrow rim along the occludent margin is slightly pro- 

 tuberant, within which there is a slight parallel depression. The scutal margin is not quite 

 straight ; the lower half (which probably was in contact with the upper latus) projecting a 

 little. 



Carina (fig. 10,/, _^) ; triangular, tapeiing, moderately strong, with a smooth surface; 

 almost straight or slightly curved inwards ; transversely, flatly arched, sub-carinated. 

 Lateral margins narrow, inflected, with the lines of growth first curved downwards (y), 

 and then on the edge itself abruptly upturned ; basal margin bluntly pointed, apparently 

 in a variable degree. The two baso-lateral angles are slightly prominent, in conformity 

 with the above-stated direction of the lines of growth along the margin. I may remind 

 the reader that this valve, the terga, and the latera presently to be described, were all 

 embedded together in their proper positions. 



Rostrum (fig. 10, h, i) : in Mr. Flower's collection, with the other loose valves from 

 Stoke Ferry, there is one which I can hardly doubt is a rostrum : it is nearly an equilateral 

 triangle ; externally convex, sub-carinated, basal margin not protuberant. The upper part, 

 for about one fourth of the entire length of the valve, must have projected freely ; this 

 part is remarkable, from having been filled up internally with a central crest, like the carina 

 of P. validus and carinatus. 



Upper latus (fig. 10, k, a) ; flat, almost an equilateral triangle, but with the two upper 

 sides not exactly equal, and both a very little longer than the basal margin ; umbo 

 of growth at the uppermost point; length, one third of the terga; surface smooth, with no 

 trace of a central longitudinal ridge ; internally the edges are bevelled, and a very small 

 portion of the apex must have projected freely. 



Lower latera (fig. 10, /, a). There are two of these (one much broken) nearly resembling 

 each other : from analogy with the latera of P. unguis, these are probably from the rostral 

 end of the capitulum ; they are slightly convex, and approximately form a transversely 

 elongated triangle, with one corner cut off ; the two upper sides are slightly unequal in 

 length : umbo of growth at the apex, from this point a narrow well-defined ridge runs 

 obliquely across the valve, to the most protuberant point of the basal margin, which is 

 situated about one fourth of the entire width of valve from one end. These lower latera 

 are wider than, but not so high as the upper latus : seen internally the edges are bevelled, 

 and a very small portion of the apex must have projected freely. 



As it is so very rare to find the valves of a PoUicipes united, it will be advisable to give 



