452 CrtETACEOUS PELECYPODA 



nEMIPLICATULA, Besh., 18G1, (see p. 451). 



1. P Hemiplicatula detrita, StoUczka, PI. XXXIII, Fig. 4, and PI. XL, 



Fig. 7. 



Hemipl. (? ) testa rotunclate ovcda, tenui, sttb-cequivalvi : valvis pernio con- 

 vexiuscnUs, dextra altera magis depressa ; timbonibus tumescentibus, aciitis, sttb- 

 medianis ; margine antico suh-umhonali leviter emarylnato, p)ostico coni'exiiisculo et 

 tnodice expanso ; sjtperficie striis coiicentricls crassinscidis, sub-lamellatis, et ad in- 

 tervalla snlcis incremeutl nonnulUs notata ; in ntroque latere tonbonis costula brecis 

 ill valois dextra ac sinistra conspicita est : costulis suh-ceqiialibiis. 



Shell roundly ovate, rather thin ; valves sub-equal, the right being a little less 

 convex than the left one ; beaks sub-central, somewhat tumid and pointed ; anterior 

 margin below the beak slightly emarginated ; posterior convex and somewhat ex- 

 panded ; surface ornamented with concentric, rather sti'ong striae and some more 

 distant undulations, indicating stages of growth. In both the valves examined 

 the shell is only partially preserved; on the cast there is a short hinge-rib in each 

 of them visible on either side of the beak, — these ribs meeting at the beak at an 

 angle of about 90 degrees ; the anterior rib appears to be a little longer than the 

 posterior. 



The general shape of the shell and the presence of hinge-ribs in both valves 

 agree with Deshayes' Hemiplicatula, but in the present species the ribs are more 

 diverging, and there is no distinct indication of the small pit in one of the valves 

 of Hemipl icatida ; as the two valves examined are, however, far from sufficiently 

 perfect, it would be to no advantage to introduce a new generic name for them. 



Locality. — Ninnyoor, in a white earthy limestone. 



Formation. — Arrialoor group. 



XLV. Family— OBTTIEIBJE. 



The organization of the Ostrece is in some resj)ects the most simple one of all 

 Pelecypoda. The mantle margins of the animals are qviite separated, with a double 

 thickened edge, more or less distinctly fringed ; the gills are simple, the leaflets 

 not being doubled on themselves, iiuited posteriorly together and to the mantle ; 

 one pair of sub-triangular, elongated, labial appendages on cither side of the mouth, 

 at the base of which they are connected with a plain membrane ; adductor muscle 

 single ; foot obsolete ; sexes distinct. 



The shells are of an elongately-ovate or of irregular shape, the former are at 

 least temporarily free, the latter always attached with the larger valve ( ?the right), 

 the left valve being usually somewhat smaller ; beaks straight, irregularly bent, or 

 spirally twisted ; ligament internal or sub-internal, placed in a longitudinal groove 

 and extending more or less at both sides of it ; hinge edentulous or sometimes with 

 an obtuse prominence below the anterior part of the ligamental area ; muscular scar 

 roundish or sub-ovate, generally excentric; jiallial line entire; structure foliated 

 externally, more or less homogeneous and sub-nacreous internally. 



