THE LAMELLIBRANCHIA 267 
CycLADIDAE, Clark. Mantle with one siphon or with two free siphons, 
which have simple orifices. _Hermaphrodite ; the embryos incubated in 
the external gill-plate. Shell with a simple pallial line. Freshwater. 
Genera—Cyclas, Bruguitre (= Sphaerium) ; two siphons ; British (Fig. 218). 
Pisidium, Pfeiffer ; asingle analsiphon ; British. FAmILy 21. RANGIIDAE. 
Mantle with two short siphons united at their bases, and with papillose 
orifices. Foot linguiform. Shell with prominent umbones and an 
internal ligament. Genus—Rangia, Desmoulins; from brackish water 
in Florida. Faminy 22. Carpinupa®, Zittel. Shell elongated, inequi- 
lateral, the posterior side being the longer; the ligament external ; the 
pallial line simple; dimyarian. An extinct family, ranging from the 
oc aie 
Fic, 240, 
Entovalva, left-side view. a, anus; c.g, cerebral ganglion ; f.g/, foot-gland; g.gl, gonad ; i.c, 
incubatory chamber; in, intestine; J, liver; m, mouth; pa, mantle; sh, shell. (After 
Voeltzkow.) 
Devonian to the Cretaceous. Genera—Cardinia, Agassiz; Trias and 
Jurassic. Anthracosia, King ; Carboniferousand Permian. Anoplophora, 
Sandberger ; Trias. Pachycardia, Hauer; Trias. Famity 23. MrEGA- 
LODONTIDAE, Zittel. Shell inequilateral, thick, dimyarian, with pro- 
minent umbones; the posterior adductor impression borne on a myo- 
phorous apophysis. An extinct family, ranging from the Devonian to 
the Cretaceous. Genera—Megalodon, Sowerby ; from the Devonian to 
the Jurassic. Pachyrisma, Morris and Lycett; Trias and Jurassic. 
Durga, Bohm ; Jurassic. Dicerocardivm, Stoppani; Jurassic. Faminy 24. 
Unronipakz, Fleming. Mantle with a single pallial suture and no 
siphons. Shell equilateral, with lateral hinge teeth or no hinge 
teeth. Inhabitants of fresh water. Development through a glochidium 
