288 'Ml'. A. Haacock on the Animal of MyochatHa siMoliiiQidei^/ 



siphonal tubes. These latter are quite distinct, though placed > 

 near together at their origin ; they are rather long and narrow> 

 the exhalant or upper tube {b) being slightly inflated towards the 

 extremity, and longer than the other {a) ; but this disparity may 

 be owing to the irregular contraction of the parts. The orifice 

 of each is fringed with a circle of simple, minute papillie ; the 

 papillae of the inhalant tube were very imperfectly observed. I 

 failed to detect the siphonal tentacles described by M. Deshayes.. 

 The fourth orifice (fig. 3 c, & fig. 1 i), which is minute and cir»; 

 cular, is situated immediately below the inhalant tube, and w ithin 

 the inferior angle of the recess formed by the margins of the 

 lobes. This orifice leads into the branchial chamber, and is un- 

 doubtedly similar to the foui'th aperture which I have pointed 

 out in Chamostrea, and in some other Lamellibranchs with closed 

 mantles ; and here, as in them, it is probably for the purpose of 

 allowing the water to escape on the sudden withdrawal of the 

 tubes and closing of the valves. The pedal orifice (fig. 1 d, & 

 fig. 4 a) is very small, and is situated far forward directly below 

 the anterior adductor muscle (fig. 4 c, c, c) which forms its supe- 

 rior wall, and around the lower extremity of which the foot 

 would seem to play. At this point, which corresponds to the 

 angle of the shell indicating the union of the anterior slope with 

 the ventral margin, the borders of the lobes {b, b) separate a 

 little, forming a similar recess to that from which the tubes 

 issue. The pedal orifice is at the lower angle of this recess, all 

 the rest of the space above being closed by the adductor muscle, 

 much in the same manner as in Chamostrea. The margins of the 

 lobes, in the vicinity of the pedal orifice and siphonal tubes, are 

 simple and grooved; along the anterior and posterior slopes 

 (fig. 1 a, b) they are closely united forming a sharp edge, and 

 following the ventral margin (e) they can be traced as two indi- 

 stinct grooved ridges closely approximating to each other. 



When the mantle-lobe is laid open, the body or visceral mass 

 {j), partially enveloped in the gills and projecting from above, is 

 found to occupy the greater portion of the branchial chamber ; 

 the pedal orifice [d) being seen at the anterior extremity, and 

 the two siphonal apertures {e,f) at the posterior; the latter 

 having the fourth minute opening (?) immediately below them. 



The mouth (fig. 2 d) is situated abo\e directly in front of the 

 visceral mass ; it is rather large, transversely oval, and guarded 

 by two pairs of palps (tig. 1 /, & fig. 2 c, e, f,f). The membrane 

 uniting the upper is ample, and forms a hood overhanging the 

 oral aperture ; the lower pair are also united at the base by a 

 membrane of considerable extent. The palps, which are of mo- 

 derate size, are wide at the base, and gradually taper to a point ; 

 the lamina on the inner surface are not numerous, there being, 



