118 Mr. H. Seeley on Cambridge Palaontology : — 



base is free, it shows indications of coarse plication. Muscular 

 impression semicircular near the left side and below the middle 

 of the shell. It is shallow, and marked with crescentic lines. 



In partially grown specimens there is scarcely any hollow, 

 the right side not being much elevated till a large size is attained ; 

 young specimens are also more elongated. In the full-grown 

 form the sides are very massive, and the attached base exceed- 

 ingly thin. The upper valve figured appears to have been a 

 feeding-ground for limpets, one having excavated for itself a 

 hollow in the shell ; and in their foraging excursions they have 

 so eroded it, that scarcely a trace is left of the lines of growth. 

 This very distinct oyster has some resemblances to O. cunabula 

 and to O. Lei/merii. From the former it is distinguished by its 

 attached mode of growth, spreading form, wedge shape, and 

 muscular impression. Its hollow shell, wedge shape, &c., suffi- 

 ciently distinguish it from the latter. If the upper part of the 

 muscular impression in it were removed, D'Orbigny's figure of 

 the interior of the loirer valve of Lei/merii might reseuible an 

 interior of the upper valve of 0. lagena. 



Very rare. Coll. University ; J. Carter, Esq. ; Mr. "W. Farren. 



Loc. Ashwell and Cambrid2:e. 



Some Exogyrse show, on weathering, a cellular structure not 

 unlike that in the principal layer of the Radiolite. 



Peden Barretti. PI. VI. fig. 1. 



Oval, slightly elongated, subequivalve, depressed. The upper 

 valve rather inflated, the lower nearly flat. The left valve is 

 ornamented with some forty-eight radiating, straight, rather 

 depressed ribs, of which about a dozen are subordinate, the 

 remainder nearly equal. All are imbricated ; the imbrications 

 are not in circles of growth. The spaces between the ribs 

 in the centre are more than half the width, and at the sides 

 more than the width of the ribs. It is on the anterior 

 side, in these spaces, that the subordinate ribs are chiefly 

 developed. The interstitial spaces are very finely striated — 

 those in the centre longitudinally, and those of the sides 

 obliquely — reminding one of P. Aptiensis. The posterior ear 

 is marked with very fine, close, sharp transverse striations ; 

 the anterior ear has two series, one radiating from the umbo, 

 the other parallel to the long diameter of the shell. On the 

 flat valve the ribs appear less prominent and more numerous. 

 There are subordinate ribs on the anterior side. All are 

 imbricated. 

 This shell approaches very near to the Pecten Espaillaci, 



D'Orb., which appears to have existed with it. The chief di- 



