INVERTEBRATE PALEONTOLOGY. 67 



beaks not terminal. Again, d'Orbigny's figures of G. linguloides represent 

 il as being laterally curved and having both valves convex, the right one even 

 more so than the left; while our shell is never laterally arcuate, and always 

 has the right valve flat and the left decidedly convex. His figures also, show, 

 as do those of Professor Forbes, that the G. linguloides has the posterior 

 basal extremity more narrowly rounded than any of the specimens of our 

 shell. These differences, which are constant in a large series of specimens 

 of the shell under consideration, when taken in connection with different hor- 

 izons at which they occur, renders it improbable that the species is the same 

 as that described by Professor Forbes. 



Locality and position. — Deer Creek, near North Platte; in the lower 

 part of the Fox Hills beds of the Upper Missouri Cretaceous. 



MYTILIDyE. 



Genus MYTILUS, Linnaeus. 



Syiion — "Perna (sp.), Adanson (1757), Senegal, 217; Retzius (1788), Diss., 20.— Schum. (1817), Ess., 120 



(not Brug. and others). 

 lli/Hlus (sp.), Liun. (1758), Syst. Nat, ed. X, 704; and (1767), ib., ed. XII, 1155.— Miiller (1776), 



Zool. Dan. Prodr., 199.— Da Costa (1778), Brit, Conch., 214.— Brug. (1792), Eucyc. 



Me'tb., I, xiii.— Lamarck (1799), Prodr. 88 (as now restricted).— Gray (1847), Zool. 



Proceed., 198.— H. and A. Adams (1857), Genera Recent Moll., 11,512. 

 Mytulus (sp.), Retz. (1788), Dissert. 



Callitriche and Callitrichoderma (sp.), Poli (1791), Test. Utr., I, 194; and (1795), ib., II, 254. 

 Aulacomya, Morch (1853), Cat. Couch. Yoldi, 53. — H. and A. Adams (1857), Goner. Recent Moll., 



II, 513 (subgen. Mylilus). 

 Hormomya, Morch (1853), Cat. Conch. Yoldi, 53. 

 Stavelia, Gray ( 1858), Proceed. Zool. Soc. Loud., 90 ; and Annals & Hag. N. IL, 3d ser., II, 62.— 



II. and A. Adams (1858), Gen. Recent Moll., 11,651 (subgen. Mytttua). 



Etym. — /ivti'ao;, a muscle. 

 Type. — Alytilus edulia, Linn. 



Shell equivalve,or very rarely inequivalve, extremely inequilateral, oblique- 

 ly subovate, taperingto the beaks, moreor less convex; posterior side compressed 

 and rounded; beaks terminal, pointed, and straight, or a little arched; sur- 

 face covered with an epidermis; hinge edentulous; ligament linear, marginal 

 or subinternal; muscular impressions unequal, the anterior being small and 

 placed near the beaks; pallial line obscurely marked. 



H. and A. Adams admit three subgeneric groups under this genus, dis- 

 tin°"uished by the following not very important differences: 



*Adanson's names being pre-Linnaean, and not binomial, cannot be properly retained. 



