CONCHIFERA. 191 
oblong; right valve with a curved, conic tooth in front of the 
oblique, sub-trigonal cartilage-pit. Animal with thick united 
siphons, fringed at the end, anal valve conspicuous ; foot finger- 
like, with a byssal groove. Distribution, 2 species. Britain, 
France. Burrowing in oyster-shells and limestone, in 10—25 
fathoms. fossil, 20 species. Tertiary. Europe. 
NERA, Gray. 
Etymology, Necera, a Roman lady’s name. 
Type, N. cuspidata, Pl. X XITI., Fig. 5. 
Synonym, Cuspidaria, Nardo. 
Shell globular, attenuated, and gaping behind ; right valve a 
little the smallest; umbones strengthened internally by a rib 
on the posterior side ; cartilage process spatulate, in each valve 
(furnished with a movable ossicle,—Deshayes), with an obsolete 
tooth in front, and a posterior lateral tooth; pallial sinus very - 
shallow. 
Animal with the mantle closed ; foot lanceolate; siphons short, 
united, branchial largest, anal with a membranous valve, both 
with a few long, lateral cirri. 
Distribution, 22 species. Norway; Britain, Mediterranean, 
Canaries, Madeira, China, Moluccas, New Guinea, Chili. From 
12—200 fathoms. 
Fossil, 14 species. Oolite—. Britain, Belgium, Italy. 
Fig. 266. Thetis, minor, Sby. Neocomian, I, Wight. 
THETIS, Sowerby. 
Etymology, Thetis, in Greek mythology, a sea-nymph. 
Synonyms, Poromya (anatinoides), Forbes. Embla (Koren), 
Lovén? Inoceramus (impressus), D’Orb? Corbula (gigantea), 
Sby. . 
Lype, T. minor, Fig. 266. T. hyalina, Pl. X-XII., Fig. 11. 
Shell sub-orbicular, yentricose, thin, translucent, surface 
regularly granulated, interior slightly nacreous; ligament (/) , 
