POLLICIPES. 71 



arched and protuberant ; an obscure rounded ridge runs from the apex to the baso-lateral 

 angle ; apex with the internal surface filled up solid, without any distinct fun'ow. 



Upper Cbalk, Norwich. Mus. fitch. 



My materials consist of two scuta, of which one is young ; and of some terga and a 

 carina which I provisionally here describe. 



Scuta (fig. 5, c), broad, moderately convex ; rather thick and strong ; surface strongly 

 ribbed from the apex to the basal margin ; ribs rather broad : apex much acuminated ; 

 occludent margin nearly straight, at right angles to the basal margin, as is also the lower part 

 of the tergo-lateral margin ; the upper part of this latter margin is inflected. A broad, very 

 slightly prominent ridge runs from the apex to the baso-lateral angle, which is broad and 

 rounded, but not prominent. There is no second ridge from the apex to the basal margin, 

 but along the line where such occurs in P. glaher, the valve is rather abruptly arched ; and 

 in the younger specimens, a distinct trace of a ridge can be seen. Internally, the pit for 

 the adductor muscle is conspicuous ; the whole upper part of the valve is filled up and 

 rendered solid ; the internal occludent edge does not widen above the adductor pit ; on the 

 tergal margin a wide internal ledge is marked by lines of growth, is slightly concave, 

 irregular, but not fun-owed ; on its surface, however, in the larger specimens, there is, 

 some little way below the apex, a small ridge and linear hollow, which probably affected, 

 in some peculiar way, the shape of the terga; but I doubt whether this structure is 

 constant. In general external aspect, this valve comes nearest to the SccdpeUum arcuatum; 

 but its tergo-lateral margin not being angularly bent, its baso-lateral angle not being 

 prominent, and the greater width of the ridges easily serve to distinguish it. 



Ter(/a (fig. 5, b ; in a reversed position, compared to the other Jit/ ures). Mr. Sowerby has 

 figured a tergum in the 'Mineral Conchology,' Plate 606, fig. 1, under the name of P.sid- 

 catus, from the Upper Chalk, and I have had given me by Mr. Woodwai'd a second specimen 

 from the Upper Chalk of Norwich. From reasons stated under my description of Scalpellum 

 maximum, var. sulcatum, I do not think it can belong to that species ; and from one trifling 

 character, namely, the kind of ridge running from the apex to the basal angle, most likely 

 it belonged to a PoUicipes ; and as only one species in the Norwich beds is as yet known 

 to be ribbed longitudinally, I believe that I have rightly attributed this valve to P. striatus. 



almo.st to the base, is marked by lines of growth, showing that the valve rather widely overlapped the terga. 

 Amongst the specimens there is one very narrow valve, curled a little laterally as well as inwards ; I believe 

 it to have belonged to a young and injured or monstrous individual. 



Affitiities. The P. dorsatus, though most readily distinguished from the P. elegans of the same Faxoe 

 formation, by the absence of ridges ou the valves parallel to the lines of growth, and likewise by the absence 

 of a strong ridge, running from the apex to the baso-lateral angle of the scuta, certainly has a considerable 

 affinity to it, as is particularly manifest in comparing the terga of the two species ; in the scuta of P. elegans 

 (fig. 9 c) the truncated end of the oblique wall-sided ridge is obviously analogous with the broad truncated 

 baso-lateral angle in P. dorsatus. This species is also related, as we have seen, to P. validus, and P. gracilis ; 

 again, also, it is related to the tertiary P. carinatus, and through it remotely to the recent P. ndtella. 



