INVERTEBRATE PALAEONTOLOGY. 23 



posterior side obliquely truncated from the beak, and very narrowly rounded 

 at its connection with the ventral border, provided with a broad, oblique, 

 rounded fold; pallial margin nearly straight, or but slightly convex ; umbo 

 marginal, rather prominent. Lower valve ? nearly flat or compressed, and 

 more irregular than the other. Surface marked by small, irregular, concen- 

 tric wrinkles, and very obscure lines of growth. 



Length, 1.37 inches; breadth, 1.14 inches. 



If the flat valve seen in connection with our specimen of this species, 

 and described above doubtfully as its lower valve, is really such, it would 

 not be a true Anomia, but would more probably belong to Placunopsis of 

 Morris and Lycett, and have to take the name Placunopsis subtrigonalis, as it 

 seems to show no traces of a perforation or sinus for the passage of a byssal 

 plug. It is true, Placunopsis was founded on Jurassic species, and is not 

 known to occur in the Cretaceous; but that is no reason why it may not occuir 

 hi rocks of that age. 



Specifically, it is rather peculiar in presenting a subtrigonal outline, with 

 a Tellina-Y\ke posterior fold or flexure. 



Locality and position. — Bijou Hill, on the Missouri ; in the Fort Pierre 

 group, or formation No. 4 of the Cretaceous. 



PECTINIDiE. 



Genus CHLAMYS, Bolten. 



Synon. — Peeten (sp.), Klein, aud other pre-Liumeau writers; and partly or entirely of numerous subse- 

 quent authors; but not of Miiller (1776), as restricted by Lamarck in 1799. 

 Argus (sp.), Poli (1791), Test. Utri Sic, 32.— Gray (1847), Proceed. Zool. Soc, 200. 

 Argodvrma (sp.), Poli (1795), Test. Utr. Sci. (uot Scopoli, 1777). 



Chlamgs, Bolten (1798), Mus. Boltenian. (2d ed., lt>19, 112).— Gray (1847), Proceed. Zool. Soc., 200; 

 as sect, of Argus.— II. aud A. Adams (1855), Gen. Recent. Moll., II, 55u (as subgeu). 

 Peeten. — Sotliczka (1871), Palseont. Ind., 111,425 (as subgeu). Peeten. 

 Etym. — xkapis, the toga, or outer garment, worn by Eomau gentlemen. 

 Examp. — Peeten Islandivus, Liuu. 



Shell closed, vertically broad-subovate, or suborbicular exclusive of the 

 ears, equivalve or subequivalve ; ears of moderate or rather large size, the 

 anterior being usually larger, and often somewhat ascending, with beneath it 

 a more or less deep byssal sinus in the right valve; surface ornamented with 

 radiating, often scaly, or transversely-striate cosfee ; hinge-line straight, 

 edentulous; ligament marginal, linear; cartilage occupying a central pit; 

 muscular scar large and subcentral. 





