108 Mr. A. Hancock on the Animal 0/ Chamostrea albida. 



by and a little anterior to the beak, the margin of the conjoined 

 mantle is deeply sinuated or notched (/) to permit of the passage 

 of the tooth. The margins of the lobes are smooth, a little 

 thickened and grooved throughout, and for some short distance 

 within are well supplied with radiating muscles which are attached 

 along the pallial line, and are for the purpose of drawing in the 

 free borders. 



The siphonal tubes (PI. III. fig. lf,ff,Sc PI. IV. fig. 2) are quite 

 distinct, and are placed a little apart; they are situated lower 

 down than usual, being almost ventral, are short and wide with 

 the walls rather thin, and with both orifices encircled by minute 

 simple papillae. Besides the siphonal apertures there are other 

 two, — one, the pedal (PI. III. fig. 1 c), has already been described ; 

 the other {m) is comparatively minute, and is situated a little in 

 front of the lower or inhalant tube, and in the angle of the 

 recess formed by the opening of the mantle-borders. A similar 

 fourth aperture exists in several, — perhaps in most of the Lamelli- 

 branchiata with closed mantles. I have observed it in Lutraria, 

 Cochlodesma, Panopeea, and Myochamay and Professor Owen has 

 described a fourth orifice in Pholadomya. It would seem pro- 

 bable that this fourth aperture is to allow the escape of the con- 

 tained water when the tubes are suddenly withdrawn and the 

 valves contracted, as always happens when the animal is startled. 

 It might be supposed that the pedal orifice would be sufficient 

 for this purpose; but it must be recollected that in all these 

 animals it is comparatively minute, and is placed towards the 

 anterior extremity. The contraction being instantaneous, this 

 fourth aperture, which is situated at the posterior end of the 

 chamber, would seem necessary to prevent undue pressure in 

 this region, while the pedal orifice will sufficiently relieve the 

 parts in front. 



The anterior adductor muscle (PI. III. fig. 1 d) is long, narrow, 

 and slightly arched outwards, and has the upper extremity 

 pointed, and rather suddenly curved inwards ; the lower ex- 

 tremity is rounded. The posterior adductor {h) is much shorter 

 and thicker in proportion to its length than the anterior, is some- 

 what arched outwards and has both extremities rounded. 



On laying open the mantle, the body (fig. 2 a) is seen sus- 

 pended from the dorsal and posterior walls of the chamber, the 

 anterior adductor [b) forming the boundary in front ; immediately 

 below this muscle is the pedal orifice (c), and in the ventral wall, 

 towards the posterior margin, project the two large, partially 

 retracted siphonal tubes [d, e), and a little in advance of them is 

 the fourth minute aperture (/) with slightly projecting margin. 

 The gills {g, (/') lie near the centre of the chamber suspended 

 fi'om above and behind ; they are large, of an irregularly rounded 



