216 UNGULINIDA. 
projecting from the base of the shell or folded up between the 
gills ; palpi pointed; gills two on each side, unequal, the external 
narrower, with a free dorsal border, inner widest in front. 
Axinopsis, Sars, 1878. 
Distr.—A. orbiculata, Sars (cxix, 69,70). Norway. 
Shell discoidal, tumid in the middle, compressed towards the 
margins; umbones slightly prominent; no external ligament ; 
valves thin, pellucid, white, concentrically striate ; cardinal tooth 
in the right valve obtusely elevated, recurved, in the left valve 
elongated, subhorizontal; cartilage narrow. 
Myst, Leach, 1819 (Brown, 1827). 
Syn.—Diplodonta, Brown, 1831. 
Distr.—40 sp. West Indies, Rio, Britain, Mediterranean, Red 
Sea, West Africa, India, Corea, Australia, California. Fossil, 
30 sp. Cretaceous, Eocene—; United States, Europe. MM. Bra- 
siliensis, Phil. :(exix, TW): 
Shell suborbicular, smooth; ligament double, rather long, sub- 
marginal ; hinge-tceth 2:2, of which the anterior in the left valve, 
and posterior in the right are bifid; muscular impressions pol- 
ished, anterior elongated. 
Animal with the mantle-margins nearly plain, united; pedal 
opening large, ventral; foot pointed, hollow; palpi large, free ; 
gills two on each side, distinct, the outer oval, inner broadest in 
front, united behind ; branchial orifice small, simple ; anal larger, 
with a plain valve. 
TENEA, Conrad, 1870. Shell roundly ovate, thin, tumid; left 
valve under the apex with a V-shaped tooth, the anterior lobe of 
which is continued along the anterior margin of the shell, sepa- 
rated by a deep groove from it; in the right valve are two 
cardinal teeth united above, the anterior is faleate, with a pit on 
each side, the posterior curved and directed obliquely backwards. 
M. parilis, Con. (exix, 72). 
FELANIA, Recluz, 1851. Shell sublenticular, equivalve, equi- 
lateral, thin, with an epidermis; beaks and lunule small; two 
divergent subapical teeth, the posterior of the right and anterior 
of the left valve channeled or bifid ; no lateral teeth, but in place 
of them a deep, long groove on each side the cardinals; liga- 
ment cartilaginous, very long ; muscular impressions oval-oblong ; 
pallial line with a short trigonal sinus. 10 sp. J. rosea, Recluz 
(cxix, 73, 74). 
Hippacus, Lea, 1833. 
Distr.—H. Tsocardioides, Lea. Eocene; Ala. 
Shell ovate, higher than long, tumid, with prominent, atten- 
uated incurved beaks; of moderate thickness; internal and 
external superficial layers of a silky appearance; hinge edentu- 
