OF SOUTHEEX IXDIA. 451 



1. TlaCHua, Brug., 1792, fPlacenta, Retzius, 1788, non Kloin, 1734 ; eadem 

 H. and A. Adams). Suborbicular, compressed, eqiiivalve, anteriorly sub-anriculate, 

 the ear being indicated by an indistinct fiu'row ; binge in the right valve with two 

 unequal, moderately-diverging ribs, the posterior being the longer one ; cartilage 

 attached to the external side of the ridges in the right valve corresponding to 

 two equally long grooves in the other valve ; ligament thin, marginal ; muscular 

 scar roundish, of moderate size, sub-central, placed somewhat posterior to the 

 median line of the hinge. Type, P. placenta, Linn., (Placenta oi'biciclaris, Retz.,) 

 the only recent species known. It not only differs by its slightly-diverging binge- 

 ribs, but also by being anteriorly indistinctly auriculate, while in Placmiema there 

 is no trace of an auricle. 



Deshayes (Paris foss., 2nd edit., vol. ii, p. 126,) discusses at length the author- 

 ship of the name Placuna, and arrives at the conclusion that the name has been 

 wrongly attributed to Solander-teste Chemnitz,* and that its true author is 

 Bruguier. The name has, however, been proposed for the type species Ostrea 

 placenta of Linne, and should, therefore, be reserved for it, as Retzius' name 

 Placenta has been pre-occupied by Klein, which fact is admitted even by 

 Deshayes, for Klein's name was published 24 years earlier than Linne's xth 

 edition of the Syst. Xaturte. 



2. Placunema, Stoliczka, 1870, (Placuna apud H. and A. Adams). Shell 

 iri'egularly sub-orbicular or sub-quadrangular, compressed, with a more or less 

 straight hinge-margin and sub-central minute, indistinct beaks, no indication of a 

 byssal sinus ; cardinal ribs in the right valve widely diverging, sub-equal ; cartilage 

 attached to the external margins of the hinge-ribs and lodged in corresponding 

 grooves of the other valve ; muscular scars roundish, below the centre of the 

 binge. Type, P. sella, Gmel. 



Only a few recent species are known. A few species have been recorded under 

 the name Placuna from tertiary deposits, but the relations of the two genera have 

 not been satisfactorily determined ; one or two cretaceous forms, wliich had first 

 been described as Placnnce, were subsequently referred to distinct genera. I 

 have adopted a new name for this genus, because the only one, Ephippimn, which 

 could be used for it has been variously applied by different authors, and has besides 

 been retained in Entomology. 



3. RemipjUcatula, Desh., 1864, (Paris foss., 2nd edit., vol. ii, p. 128). Shell 

 roundly oval, solid, compressed, sub-equivalve, binge Avith two slightly diverging 

 binge-ribs in each valve, those of the right valve fitting between those of the 

 left, which are less elevated and have between them a small fosset ; the cartilage 

 is attached, as in Placuna, along the external sides of the hinge-ribs, and this forms 

 the principal distinction between the present genus and Plicatula, where the car- 

 tilage is situated in the median pit. Type, Hemipl. solicla, Desh. (Olim Placuna 

 solidaj, from the Paris basin. 



The genus represents a connecting link between Placuna and Plicatula ; the 

 umbones are slightly flattened and may have been temporarily attached. 



* I am unable to refer to tlie passage in the old edition of Cliemnitz. 



