964 MYTILIDA. 
SuBraAMILy CRENELLINA. 
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. 
Htym.—Diminutive of crena, a notch. 
Syn.—Myoparo, Lea, 1833. Stalagmium, Conr., 1833. 
Disir.—5 sp. Low-water mark to 150 fathoms. Norway, 
Iceland, Greenland, New England, Britain, France. Fossil. 
Eocene; Ala. CU. rhombea occurs in a fossil state in the Coralline 
Crag, England. C. decussata, Mont. (exxix, 17). 
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 Modiolaria, 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. C. divaricata, d’Orb. 
(exxix, 18), 
pacripiuM, Torell, 1859. Hinge-crenulations tuberculiform 
anteriorly, elongate posteriorly. D. vitrea, Sars (cxxix, 19). 
MopronartA, Beck (Jeffreys, 1863). 
Htym.—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. WM. 
impacta, Herm. (exxviii, 99). 
Shell rhomboidal, sculptured by two rows (one on each side) 
of striz, which radiate from the beaks, leaving the middle portion 
smooth, umbones incuryed, 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. 
ARCOPERNA, Conrad, 1865. 
Distr.—Kocene; Mississippi, Paris Basin. A. filosa, Conrad 
(cxxix, 20). 
