INVERTEBRATE PALAEONTOLOGY. 69 



Bf y t i 1 n s subarcnatus, M. & H. 



Plate 38, figs. 2, a, b. 

 Mytxlui iubarcuatus, Meek and Hayden (1856), Proceed. Acad. Nat. Sci. Philad., VIII, 276. 



Shell narrow subovate, oblique, somewhat arcuate, moderately convex; car- 

 dinal and ventral margins converging at an angle of about 40° toward the beaks, 

 which are pointed and nearly straight; posterior extremity wider, compressed, 

 and rather narrowly rounded below; hinge about half the length of the shell, 

 a Utile arched, and passing gradually into the carinate postero-dorsal margin, 

 which descends, with a regular convex curve, to the lower part of the posterior 

 extremity; antero-ventral margin very slightly concave in outline. Surface 

 unknown. 



Length, 1.03 inches; height, measuring at right angles to the longer axis, 

 0.36 inch; convexity, about 0.28 inch. 



This species resembles M. lanceolatus of Sowerby (Min. Con., pi. 439, 

 fig. 2), but is proportionally wider behind, and not cpiite so pointed at the 

 beaks It seems to be about intermediate in form between that species and 

 M. edentulus of the same author. The only specimen we have seen is a cast, 

 showing none, of the surface-characters, nor the muscular impressions. 



Locality and position. — Dog River, near the mouth of Judith River; 



where it was found in a Cretaceous sandstone belonging to the latest beds of 



the Fox Hills group. 



Genus VOLSELLA, Scopoli. 



Synon. — Perna* (sp.), Adanson (1757), Senegal, 207. — H.aud A. Adams (1857), Genera ReceutMoli., II, 515. — 

 Courad (1862), Proceed. Acad. Nat. Sci. Philad., XIV, 579; and (18C5), Am. J. Conch., 



I, 10 (not Perna, Retzius, Brug., and others). 



rohella, Scopoli (1777), Intr. Hist. Nat., 397 ..-Modeer (1793), K. Vet. Ac. Haudl., 392.— Gray (1847), 



Proceed. Zool. Soc, 197. — Meek and Hayden (1864), Palseont. Upper Mo., 85. 

 Tumarindiformis (part), Mensch (1787), Mas. Gevers, 412. 



Callitriche and Gallitrichoderma (sp.), Poli (1791), Utr. Sic, I, 194; and (1795), iB., II, 254. 

 Modiolus, Lam. (1799), Prodr., 87.— Cnvier (180(1), Auat, Comp., x; and (1817), Regne An., II, 471 ; 



also, ib. (1830), III, 136— Link (1807), Rost. Summl., 14C— Goldf. (1820), Zool., 611.— 



Forbes (1838), Mai. Mini, 43. 

 Modiola, Lata., (1801), Syst. An.. 113; and (1805), Ann. duMus., VI, 119; and (1819), Hist., VI. 109.— 



Fernss. (1821), Tab. Syst., xlii. — Bronn (1837), Leth., 355 ; and of numerous other later 



authors. 

 Amygdalum, Miihlf. (1811), Entw., G9. 

 P>rachydontes, Swainsou (1840), Malac, 384 (subgenus).— H. &. A. Ad. (1857), Gen. Recent Moll., 



II, 517. 



Etym. — VoUella, forceps or tweezers. 

 Type. — Mytilus modiolus, Linn. 



Shell transversely suboval or oblong, very inequilateral ; surface smooth, 

 concentrically striated, or with radiating or divaricating striae or costa 1 , and 



" Aute-Liuuaean. 



