14 MR MACDONALD ON THE ANATOMY 



The Mantle, Respiratory, and Circulatory Systems. — The free border of the 

 mantle (Plate II. fig. 1, n) is often beautifully fimbriated, and beset with cilia, 

 both stationary and vibratile ; and in the extended state, an angular fold of 

 it (Plate II. fig. 1, o) occupies the dorsal slit of the shell, and extends for some 

 little depth between the laminae at the base of the keel. 



The branchiae consist of a variable number of clavate bodies (Plate II. fig. 1, 

 p), in single longitudinal series floating from the dorsal wall or roof of the mantle; 

 and to the left of these, but passing forwards more obliquely, there is a narrow, 

 strongly ciliated band (Plate II. fig. 1, q), in all probability in some way con- 

 nected with respiration, when the animal is retracted into its shell. 



Large venous sinuses (Plate 11. fig. 1, r), distinctly observable between the 

 layers of the mantle, appear to convey the return blood from the gills to the auricle 

 of the heart (Plate II fig. i, w). The extent of this chamber is very much less 

 defined than that of the ventricle (Plate II. fig. 1, y), which communicates with 

 it by a large opening, guarded by a couple of valves. The muscular bundles of 

 the ventricular walls are also much stouter than those of the auricle, and form a 

 close interlacement, in the interstices of which the lining and investing mem- 

 branes seem to meet together. 



The great vessel arises posteriorly from the apex of the ventricle, and forms a 

 short dilated axis (Plate II. fig. 1, a), from which two arterial trunks originate ; 

 one of these (Plate II. fig. 1, /5) passes backwards to supply the abdominal viscera, 

 while the other (Plate II. fig. 1, z) runs forwards beside the oesophagus and the 

 root of the swimming-plate, divides into the pedal and buccal arteries, in every 

 respect conformable with those already described in Firoloides. 



Genei-ative System. — It is very remarkable that there is, as nearly as possible, 

 an equal distribution of males and females in the genus Firoloides, while the pro- 

 portion of males to females in Atlanta is so very great, as to render it difficult to 

 form a correct estimate. In my own experience, out of many hundreds of 

 Atlanta', I have only met with about twenty females. 



The follicles of the testicle (Plate II. fig. 1, somewhat resemble those of the 

 liver, but they are at once distinguished by their lighter colour and the nature of 

 their contents, which usually present a finely granular basis, with fasciculated stria? 

 in the axis of every cavity and passage. The whole organ, though very similar in 

 shape, is much smaller than the liver, between which and the inner wall of the 

 shell-tube it lies. A stout vas deferens leads forwards from the base of the 

 gland, and soon forms a fusiform enlargement (Plate II. fig. 1, 8) coated Avith 

 black pigment ; and from the small fore-part of which the duct still passes for- 

 wards a little way, and then terminates in a leaf-like expansion (Plate II. fig. 1, 

 v), having a fine cellular structure in the space between the adductor-muscle 

 (Plate II. fig. 1, n) and the rectum (Plate II. fig. 1, x). 



The external male organ is situated at the base of the neck, and may be 



