ORDER II TELEODESMACEA 407 
Family 15. Tancrediidae, Fischer. 
Shell Donaciform, equivalve, with an external ligament ; the margin of the valves 
entire; hinge with posterior 
and anterior laterals, the latter 
7nconstant ; cardinals one in 
the left or two in the right 
valve, or two in each valve. 
Trias and Cretaceous, ? Recent. 
Tancredia, Lycett (Hettan- 
gia, Terg.; Palaeomya, Zitt. 
and Goub), (Figs. 737, 738). 
Shell sub-arcuate, attenuated 
before the beaks, wider and iio ; ; mya) corallina, Zitt. 
; ‘ : Tancredia securiformis, Dunker sp. Lower and Goub. —Coral- 
shorter behind them ; obliquely tLias; Hettingen, Lorraine. 1/; (after Terquem). Rag; Glos, Calvados. 
truncate and somewhat gaping 
posteriorly ; ‘a. cardinal tooth on each side, and also an elongated posterior lateral. 
Trias to Cretaceous ; maximum in Laas. 
? Meekia, Gabb. Cretaceous; California. ? Hemidonaz, Morch (Donacicardium, 
Vest). Recent. 

Fic. 739. 
Tancredia (Palaeo- 

Fic. 738. 
Family 16. Unicardiidae. Fischer. 
Shell cordiform, equivalve, closed, concentrically striated ; adductor scars elliptical, 
the anterior longer; pallial line simple; margin ‘of 
the valve smooth; ligament external, parivincular, seated 
in a groove; with a grooved hinge plate bearing a single 
obsolete cardinal in each valve, or none. Carboniferous to 
Cretaceous. 
Unicardium, WVOrb. (Fig. 740). Rounded, inflated, 
with incurved beaks; hinge margin thin, with a weak 
cardinal tooth; ligament deep-seated. Trias to Cre- 
taceous. 

Fic. 740. Sealdia, Ryckholt. Carboniferous. 
_Unicardium excentricum, WOrd. Pseudedmondia, Fischer. Ligament completely external. 
Kimmeridgian ; Cap de la Heve, ,, er : 4 
near Havre. 1/,. Carboniferous. 
Family 17. Lucinidae, Fleming. 
Shell substance porcellanous or chalky, usually with inconspicuous or dehiscent 
epidermis, rounded, variably sculptured ; valves equal, free, closed, with low, prosocoelous 
beaks ; adductor and pedal scars adjacent or distinct, the latter small; anterior adductor 
elongated, largely within the pallial line, which is not sinuate ; area within the pallial line 
often granular or punctate ; cardinal area small, often deeply impressed ; ligament and 
restlium sub-internal, set in a deep groove, but usually more or less visible externally ; 
hinge plate distinct ; lateral laminae distant from the cardinals, anterior and posterior 
in the right, with corresponding sockets in the left valve; cardinal teeth radial, formula 
L1010 
RO10V’ 
or absent. Silurian to Recent. 
the posterior tooth larger and often bifid, but any or all of the teeth may be obsolete 
Paracyclas, Hall. Rounded, thin-shelled, concentrically striated; no lunule ; 
hinge unknown. Devonian. 
