334 CRANIID^. 



? AuLACORHYNCHUS, Dittmar, 1812. 



Distr. — A. concentrica^ Sem. Garb. ; 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. 



[Calceola, Lamarck, 1809. 

 This is a genus of Coelenterata.] 



Order LYOPOMATA. 



(Pleuropygia, Broun. Inarticulata, Huxley.) 



Arms free, unsupported by shelly apophyses ; intestine opening 

 by a lateral anus (Tretenterata, King) ; iDorders of the mantle- 

 lobes entirely disunited ; brachia without a distinct median lobe. 

 Shell in most cases without hinge-teeth, articulation or cardinal 

 process. 



Family 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, dii'ected towards the concavity 

 of the dorsal valve, and suppoi'ted 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. 



Crania, E-etzius, ITSl. 



Etym. — Kraneia, capitate. 



Syn. — Criopus, Poll, 1191. Orbicula, Cuvier, 1*198. 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 — ; Europe. 



Shell smooth or radiatelj^ si ated ; umbo of dorsal valve sub- 

 central ; of ventral valve subct tral, marginal, or prominent and 

 cap-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. 



