450 CEETACEOUS PELECYPODA 



cliano-es vary greatly with the state of preservation of the surface. A closely allied 

 Buvo-pcan siiecics is Spondi/lus fflobitlosus, d'Orh., only differing from the Indian 

 form bv a little more distant radiating ribs and comparatively larger ears. 



Localities. — East of Parally, Moraviatoor, Odium, in yellowish compact or 

 sandy limestone. 



Formation. — Ootatoor group. 



XLIV. Family —TLACVNIBJS. 



The animal of the typical species, Placima placenta,* Linn6, is symmetrical; 

 it has the mantle margins perfectly separated, surrounded with smaller and larger 

 cirri, and internally furnished with a kind of a pendent border, as in most species 

 of the Pectinid^ : one pair of sub-equal gills on each side, long, crescent-shaped, 

 posteriorly attenuating to a point and united ; adductor muscle moderate, sub-cen- 

 tral, round, single ; at the posterior side of it is a small, indistinct impression caused 

 by the branchiopallial muscle, and near the hinge a third, also small, but more dis- 

 tinct, impression of the pedal muscle ; foot sub-cylindrical, tubular, and expansible ; 

 lips large, sub-triangular ; ventricle of the heart free ; generative organ and rectum 

 attached to the right mantle-lobe. 



Shell equivalve, or very nearly so, compressed, thin, of a peai-ly tubular 

 structure, externally becoming often finely lamellar ; ligament marginal, cartilage 

 attached to the external side of two diverging ribs in one valve, corresjionding 

 to two similar grooves or ribs in the other valve ; muscular scar sub-central, round. 



Gray more than 20 years ago suggested the separation of Placiina and its 

 allies into a distinct family ; but since the examination of the animal of PI. placenta 

 by Woodward, it has been thought better not to separate them from the Anomiidm. 

 The organization of both is no doubt very similar, but we find that one of tlie 

 most important characters of the Anomiidje, the peculiar development of the byssal 

 muscles, is altogether wanting in the Placunid/e ; the shells of this last family 

 are, besides, symmetrical, equivalve, or sub-equivalve, free, supplied with a marginal 

 ligament and internal cartilage. Tlie habitat is also very diflferent. Placuna is 

 found on sandy shores, and has a very extensible vermiform foot, with which it can 

 bury itself partially in the sand, spinning at the same time a few threads of byssus. 

 Placunema I found loosely lying on coral-reefs. The third genus referred to this 

 family is based upon a peculiar fossil shell from the eocene of Paris, for which 

 Deshayes suggested the name Hemiplicatula. Of this last genus, an aberrant 

 form occurs in the South Indian cretaceous deposits ; it AA'ill be noticed subse- 

 quently. Of the former genera, a few fossil species have been noticed from tertiary 

 deposits only, but none from older formations. Eor one or two noted from creta- 

 ceous rocks a new generic name has been proposed by Conrad, and the Jurassic 

 species have been named Placunopjsis ; both are referable to the Anomiidje. 



* See Woodward in Ann. and Mag. Nat. Hist., 2nd ser., xvi, p. 25. 



