292 DESCRIPTION OF A COLLECTION OF FOSSILS, 



base most prominent directly under the posterior end of the hinge line. Area 

 short, narrow, and, at least in part, marked in the usual manner of the Areas. 

 Shell substance thin ; surface marked by concentric lines of growth, crossed by 

 some fine obsolete radiations anteriorly ; inner margin entire ; a very small plate 

 borders the inner edge of the posterior muscular scar. 



Figures. Natural size. 



Locality. From the hacienda of Macanga, and from the vicinity of Ollon, from 

 a rock which Dr. Raimondi considers Cretaceous. 



Remarks. In referring this shell provisionally to Area I have done so in the 

 full conviction that it will be placed in an, as yet, undescribed genus as soon as 

 more material shall have been obtained. It has the form of an oblique Limopsis, 

 but I have been able to see about the anterior third of the area, which shows two 

 or three of those incised lines which are characteristic of the Areas and Cucullseas. 

 The middle of the area not being visible, I cannot assert that it has not also the 

 Limopsis fosset, or at least a rudiment of it. The shell is unusually thin for the 

 family. It wants the marginal crenulation generally seen, and it possesses a rudi- 

 ment of the internal plate of Cucull(Ba, in the shape of a little ridge bordering only 

 the anterior margin of the posterior muscular scar, and succeeded, in the usual 

 position of the Cucullcea plate, by a very faint broadly rounded thickening which 

 runs up into the beak. It will be seen that we have here enough material to cha- 

 racterize this as a new genus, all except the hinge teeth, and the details of the 

 area, the latter of which could, however, be pretty safely inferred. 



Another little Arcoid, of which I have only seen one imperfect valve, occurs in 

 the black shale of Pariatambo. 



NUCULA, Lam. 



N. Peruana, n.s., PI. 41, fig. 1. 



Shell very small, cuneiform; beaks placed a third of the length from the ante- 

 rior end, which is rounded; posterior end narrow and rounded; base nearly 

 straight; posterior cardinal margin sloping; surface marked by fine lines of growth. 



Locality. Near Tingo ; Prov. of Huari ; Dept. of Huaraz. Associated with 

 the undetermined Trigonia and various species characteristic of the Liassic rocks 

 of Pariatambo ; also a single specimen from the latter locality. 



Remarks. A cast, showing the outline and characteristic hinge, from the former 

 locality, and another specimen retaining the surface, from the latter. From being 

 almost exactly of the same size, and having about the same degree of convexity as 

 Corhula niiculoides, with which it is associated, this shell may be easily confounded 



