452 CEETACEOUS PELEOYPODA 



HEMIPLICATULA, JDesJi., 1864, (see p. 451). 



1. ? Hemiplicatula deteita, StoUczka, PL XXXIII, Eig. 4 a^d PL XL, 



Eig. 7. 



HemipL (?) testa rotundate ovata, tenui, suh-(Bquivalvi : valvis paulo con- 

 vexiusGulis, dextra altera magis depressa; umbonibus tumescentibm^ acutis, sub- 

 medianis ; margine antico sub-umbonali lemter emarginato, postico convexiusculo et 

 modice expanso ; superficie striis concentricis crassimculis, sub-lamellatis, et ad in- 

 tervalla sulcis incrementi nonnullis notata ; in utroque latere umbonis costula brevis 

 in valvis dextra ac sinistra conspicua est : costtdis sub-^squalibus. 



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 strong 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' Semiplieatula, 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 Hemiplicatula ; 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,— OS TBmJDJEI. 



The organization of the Ostrece is in some respects the most simple one of all 

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

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

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

 one pair of sub-triangular, elongated, labial appendages on either 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-ov^te, generally excentric; pallial line entire; structure foliated 

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



