ACEPHALA LAMELLIBKANCHIATA. 27 



extended into siphonal tubes; the external surface lines the 

 shell, and is the constructor thereof, by the exudations of 

 calcareous matter from its vessels ; the inner surface embraces 

 the body, and is hung and this observation introduces those 

 on the muscular system by some inconsiderable muscles ; but 

 the grand supports of its position, in all bivalves, are the ad- 

 ductor muscles which pass through and suspend the body by 

 being fixed to each valve, and when detached leave cicatrices, 

 termed muscular impressions, which have been applied to assist 

 generic and specific determinations. It is said that in the 

 Anomice, Ostrea, and Pectines, there is only one subcentral 

 muscle : this is a mistake ; as in those genera, as well as in 

 the Mytili and some others, I have found a second muscle, 

 though often of very inconsiderable volume, under the beaks ; 

 we may therefore consider the monomyal and dimyal charac- 

 ters as untenable. The mantle in the Anomia, Ostrea, and 

 Pectines is entirely open from the beaks, and destitute of 

 siphonal tubes ; in others, as in Lucina, Mytilus, &c., it is 

 more closed, and shows only two sessile orifices ; in the fami- 

 lies of the Veneridce and Mactridte a still further closure is 

 apparent in that membrane, which in them is produced into 

 two short tubes ; in the Tellinidce it forms rather long slender 

 siphons ; and finally, in the Pholades, Lutraria, My&, Gas- 

 trochcence, and the Saxicavce, it is entirely closed, except a 

 pedal orifice, and is often produced into two long tubes, either 

 soldered on each other or inclosed in a retractile sheath ; the 

 lower siphon is called the branchial, the upper the anal one ; 

 the former conveys water to the branchial cavity and expels 

 it. We do not believe the doctrine that the water enters 

 by one tube and is discharged from another; we think it 

 is admitted both from the branchial siphon and pedal orifice, 

 and expelled indiscriminately from those apertures. In many 

 genera both, and in others only one of the siphons are pro- 

 vided with valves at their orifices, to regulate the entry and 

 expulsion of the water, and, with one or more circles of ten- 

 tacular filaments, to capture and transfer the animalculse 

 into the branchial cavity : on this point see our observations 

 on the Pholades. 



