48 Records of the Indian Museum. [Vol. XIX, 



aperture. It ma^^ extend posteriorly both along the dorsal and 

 ventral margins but specially along the latter. In two species, so 

 far studied, the free anterior margins of the mantle lobes have 

 given rise to two muscular flaps from their inner surfaces which 

 serve to close the pedal aperture when the foot is withdrawn into 

 the mantle chamber. The flaps may be named the pedal valves. 

 The outer surface of the thick muscular anterior margin of each 

 mantle lobe sometimes presents a groove parallel to and slightly be- 

 hind the free margin which receives a ridge from the valve. Pos- 

 teriorly, the mantle lobes leave two apertures one above the other 

 and placed at the end of two tubes formed from the mantle lobes. 

 They are known as the anal and branchial siphon. The two siphonal 

 tubes are either concresced completely in a single piece as in the 

 typical genera, or completely or partially separate from each other 

 in others. A fourth aperture has been described in the ventral mar- 

 gin of the fused mantle lobes. Bloomer (8, pp. 43-45) has conclu- 

 sively shown that this aperture in the Solenidae is not homologous 

 with a similar one in other Pelecypoda and is derived secondarily 

 from the posterior end of the pedal aperture. The degree of fusion 

 of the mantle lobes appears to have some distinct bearing on the 

 assortment of the genera. Lastly, from the union of the mantle 

 lobes with each other or with the walls of the siphons, spaces may 

 be formed either demarcated or completely separated from the 

 general mantle chamber. Thus a space has been described by 

 Bloomer (7) in Cultellus pellucidus formed in the dorsal aspect 

 anteriorly by the mantle lobes being joined together a little below 

 their margins. This chamber does not communicate with the 

 mantle cavity and is known as the suprapedal chamber [Bloomer 

 (8), p. 41]. Another chamber may be formed by the union of the 

 dorsal margins of the mantle lobes in front of the anterior adductor 

 muscle and lying above the foot. In some genera a space is 

 enclosed posteriorly by the mantle lobes (which are free at their 

 posterior margins), and limited in front by the union of the siphonal 

 tubes at their anterior margins with the mantle lobes, either 

 directly along their inner side in front of their free posterior margins 

 [Solecurtus, Azor), or through the interyention of narrow {Novacu- 

 lina) or wide lateral processes (Tagelus), extending from the free 

 posterior margins of the mantle lobes to the base of the siphonal 

 tubes. Further, in Tagelus and Azor, the dorsal wall of the upper 

 siphonal tube is connected with the ventral aspect of the posterior 

 adductor muscle. This space contains the siphonal tubes (which 

 may be completely retractile within the chamber) and may be 

 designated as the siphonal space. 



Pai,i,iai, Muscui^ature. a cruciform muscle is found in some 

 genera, showing their relation to the fam. Psammobiidae. 



Foot. The foot is an elongated organ more or less cylindri- 

 cal in typical forms but somewhat flattened in others. It arises 

 from the anterior or antero-ventral aspect of the visceral mass. 

 The shape of the foot, specially its anterior portion, is so variable 

 in different degrees of retraction that it is difficult to form an idea 



