264 MYTILIDiE. 



Subfamily GEE NELL IN^. 



Shell elongately tumid, thin, with subterminal slightly swollen 

 beaks, two muscular scars, of which the posterior is larger, outer 

 surface of valves entirely or partially radiately striated (except 

 in Myrina) ; hinge-line often denticulate ; ligament almost quite 

 internal, in a linear groove, more or less extending posteriorly. 



Crenella, Brown, 1827. 



Etyvx. — Diminutive of crena, a notch. 



Syn. — Myoparo, Lea, 1833. Stalagmium, Conn, 1833. 

 ' Distr. — 5 sp. Low-water mark to 150 fathoms. Norway, 

 Iceland, Greenland, New England, Britain, France. Fossil. 

 Eocene ; Ala. C. rhombea occurs in a fossil state in the Coralline 

 Crag, England. C. decu8><ata, Mont, (cxxix. It). 



Shell oval or rhomboidal, nacreous, cancellated ; umbones 

 straight, ligament small, hinge of each valve furnished with an 

 upright tooth, which is crenulated, as well as the hinge-plate. 



Animal with the mantle open in front, and folded behind into 

 a sessile excurrent tube ; foot cylindrical, the free end being 

 disk-like and issuing out of a sheath. The animal does not spin 

 a thick byssus,like Modiolarla,but secretes only a single thread 

 for attachment, and by means of which it holds itself suspended 

 in the water. ' 



NUCULOCARDIA, d'Orb., 1843. Shell with large anterior crenate 

 teeth, and smaller posterior ones. G. divaricata, d'Orb. 

 (cxxix, 18). 



DACRiDiTJM, Torell, 1859. Hinge-crenulations tuberculiform 

 anteriorly, elongate posteriorly. B. vitrea, Sars (cxxix, 19). 



MoDiOLARiA, Beck (Jeffreys, 1863). 



Etym. — Allied to the genus Modiola of Lamarck. 



Syn. — Lanistes, Humphreys. Lanistina, Gray. 



Distr. — Temperate and Arctic seas. The four British species 

 occur fossilized in the Red and Coralline Crags and newer Ter- 

 tiaries. Several species in the Upper Triassic and Jurassic 

 formations, referred to Modiola, appear to belong here. 3L 

 impacta, Herm, (cxxviii, 99). 



Shell rhomboidal, sculptured by two rows (one on each side) 

 of striae, which radiate from the beaks, leaving the middle portion 

 smooth, umbones incurved, hinge edentulous or crenulated, 

 hinge-plate finely notched. 



Animal with the mantle folded in front into a wide incurrent 

 tube, and behind into a conical excurrent tube ; foot strap- 

 shaped. 



Argoperna, Conrad, 1865. 



Distr. — Eocene; Mississippi, Paris Basin. A. jfilosa, Conrad 

 (cxxix, 20). 



