334 CRANIIDZ. 
? AULACORHYNcHUS, Dittmar, 1872. 
Distr.—A. concentrica, Sem. Carb.; Russia. 
Shell thin, broad, concavo-convex, with straight hinge-margin ; 
beak of the large valve strongly curved; no area or deltidium ; 
hinge toothless; muscular impressions small, not well marked ; 
surface leafy, without spines; large valve with a long, two-parted 
lamella, three-parted at the end, commencing at the beak and 
only united by the lateral margins with the shell. 
[CatceoLa, Lamarck, 1809. 
This is a genus of Ceelenterata. | 
OrverR LYOPOMATA. 
(Pleuropygia, Bronn. Inarticulata, Huxley.) 
Arms free, unsupported by shelly apophyses; intestine opening 
by a lateral anus (Tretenterata, King); borders of the mantle- 
lobes entirely disunited ; brachia without a distinct median lobe. 
Shell in most cases without hinge-teeth, articulation or cardinal 
process. 
Famity CRANIID/. 
Shell orbicular, calcareous, hingeless ; attached by the umbo, 
or whole breadth of the ventral valve, rarely free; dorsal valve 
limpet-like; interior of each valve with a broad granulated 
border ; disk with four large muscular impressions, and digitated 
vascular impressions ; structure punctate. 
Animal with free*spiral arms, directed towards the concavity 
of the dorsal valve, and supported by a nose-like prominence in 
the middle of the lower valve; mantle extending to the edges 
of the valves, and closely adhering; its margins plain. 
OrAnIA, Retzius, 1781. 
Etym.—Kraneia, capitate. 
Syn.—Criopus, Poli, 1791. Orbicula, Cuvier, 1798. Orbicu- 
larius, Dumeril, 1806. Chonionora, Schauroth, 1854. 
Distr.—5 sp. Spitzberger Britain, Mediterranean, India, 
New South Wales; 150 fath as. Fossil, 37 sp. Lower Silu- 
rian—; Hurope. 
Shell smooth or radiately st ated; umbo of dorsal valve sub- 
central; of ventral valve sube ‘tral, marginal, or prominent and 
-ap-like, with an obscure triai ular area traversed by a central 
line. 
The large muscular impressions of the attached valve are some- 
times convex, in other species deeply excavated; those of the 
upper valve are usually convex. 
