410 MOLLUSCA SUB-KINGDOM VI 
Diplodonta, Bronn (Fig. 748).  Thin-shelled, orbicular, convex, concentrically 
striate or pustulose ; cardinals 2. 2, the left anterior and right posterior bifid ; laterals 
a obscure or absent. Tertiary and Recent. 
Ungulina, Daudin, Nestling and _ often 
irregular. Felania, Récluz. Shell compressed ; 
feeble laterals present. Axinopsis, Sars; Sphae- 
rella, Tenea, Conrad. Cretaceous to Recent. 

Fra. 748. Family 20. Cyrenellidae. Fischer. 
ree eer ae Peas eile a uoceng Shell as in Diplodonta, but with «@ con- 
spicuous epidermis; pallial area smooth; pallial 
line not sinuate; hinge without lateral laminae; the cardinals like Diplodonta, 
or with two cardinals in each valve soldered to each other dorsally ; cardinal formula 
L010101 
R101010’ 
brackish water. 
the anterior left cardinal usually obsolete. Pliocene to Recent, in fresh or 
Cyrenoidea, Joannis (Cyrenella, Desh.) Pliocene of Florida, Recent in the Antilles 
and West Africa. 
Joannisiella, Dall. Hinge as in Diplodonta, resilium immersed, the larger cardinal 
L1010 
bifid, the teeth not soldered above. Cardinal formula hou Recent ; Philippines. 
Family 21. Cryptodontidae. Dall. 
Shell substance earthy, with inconspicuous epidermis and prosocoelous beaks ; valves 
equal, free, closed, with plain margins, smooth, or with feeble 
concentric striae, and usually with a radial posterior flexure ; 
adductors Lucinoid, pallial area often punctate ; ligament and 
resilium parivincular, opisthodetic, sub-external, seated in a 
groove ; area impressed ; hinge feeble without lateral laminae, 
edentulous, or with an obsolete cardinal tooth in the right valve. 
Cretaceous to Recent. 

Cryptodon, Turton (? Axinus, Sow.; Conchocele, Gabb), 
(Fig. 749). Smooth, thin-shelled, living in deep water. i 
Cretaceous to Recent A, Cryptodon sinuosus, Don. 
safe f a ‘ c Miocene ; Grund, near Vienna. 
Philis, Fisch. Lunule deeply indented, projecting spoon- 1). B, C. wnicarinatus, Nyst. 
° . . . Oligocene Septaria - clay) ; 
like into the cavity of the valves. Recent. Proce wade wee Benint y) 
Superfamily 8. LEPTONACEA. Dall. 
The incurrent and excurrent openings between the mantle lobes at opposite ends of the 
body, the former anterior. 
This group contains a great many commensal, nestling, or parasitic forms ; if indepen- 
dent usually very active, crawling like Gastropods on a sub-reptary foot, and with the mantle 
edges more or less reflected over the valves. 
Family 22. Leptonidae. Dall. 
Shell cellulo-crystalline with a periostracum ; valves equal, free, smooth-edged, often 
gaping, variably sculptured ; adductor scars peripheral, sub-equal ; pallial line simple ; 
area obscure or none; ligament parivincular, opisthodetic, external, often obsolete ; 
resilium usually internal, sub-wmbonal or oblique; hinge plate narrow, channelled to 
receive the resilium; hinge variable, normally consisting of one or two radiating 
