74 FOSSIL CIRRIPEDIA. 



only rendered visible or prominent by disintegration ; the transverse plaits are also rendered 

 more prominent by disintegration. 



Scutum (Tab. IV, fig. 7, d,f) elongated, triangular, but vi^ith the baso-lateral corner 

 produced; convex, with the upper part almost semi-conical; apex much pointed, and 

 curved towards the carina ; occludent margin considerably arched ; basal margin short, 

 forming, with the occludent margin, an angle considerably above a rectangle, and therefore 

 causing tlie baso-lateral angle to be much produced. The tergo-lateral margin is in the 

 upper part slightly concave, in the lower part rounded and protuberant. The baso-lateral 

 angle is broad and rounded, but with a small, central, square-sided prominence, formed by 

 the projection of the ridge running from this angle to the apex. This ridge is very con- 

 spicuous, it is narrow, being not above one third of the width of an average zone of growth, 

 increasing very little in width downwards ; it is wall-sided, that is, has its sides absolutely 

 perpendicular ; its summit is surmounted by transverse plaits, really continuous (but not 

 at first appearing so) with those on the surface of the valve on both sides of it. This ridge 

 runs in a curved line, nearly parallel to the occludent margin. Internally (/), the occludent 

 edge is broad and flat, and is marked with lines of growth ; it becomes wide at the apex ; 

 there is a rather deep furrow close to the tergal margin, but there is no trace of a central 

 ridge ; the pit for the adductor muscle lies quite close under the furrow and flat occludent 

 edge ; the nearest approach to this structure of the under side of the apex of the scuta, is 

 in the P. glaber. 



Terga (fig. 7, c). On this valve the plaits, in specimens which have not undergone any 

 disintegration, are much less strongly pronounced than on the scuta and carina; in shape, 

 sub-rhomboidal, not quite flat ;, a curved, wall-sided, narrow ridge (like that on the scutum) 

 runs from the pointed, slightly curved apex to the sharp basal angle, and itself projects 

 as a little point, with parallel sides; the growth-plaits extend across this ridge, which 

 runs at about one third of the entire width of the valve from the carinal margin. The 

 upper carinal margin is very slightly longer than the lower carinal margin, with which it is 

 almost blended by a continuous curve. The occludent is shorter than the scutal margin ; 

 the edge of the shell close to the former is rounded and protuberant, and parallel to this 

 rounded edge the valve is slightly depressed, and correspondingly slightly hollowed out on 

 the scutal margin ; internally there are lines of growth along the upper carinal and occlu- 

 dent margins. Carina (fig. 7, a, b, c) widening rather slowly from the apex to the base ; 

 almost semi-cylindrical ; very slightly bowed inwards ; not in the least carinated ; basal 

 margin not at all protuberant. The transverse plaits are, in the three specimens which I 

 have seen of the carina, undulatory ; on the very narrow lateral margins (b) the plaits are 

 obhquely upturned. The upper j)art of the carina projected freely ; this part, and both 

 margins, are internally marked by lines of growth. 



Dimensions. Most of the specimens are rather small, but I have seen one tergum seven- 

 tenths of an inch in length. 



Affinities. This species, with the two following P. fallax and elegans, form a little group 



