AND CLASSIFICATION OF THE HETEROPODA. 9 



(Plate I. fig. 4, s) divide and interlace with one another, leaving angular and irre- 

 gular spaces between them. They are not so clearly to be traced in the ventricle 

 (Plate I. fig. 4, t), the walls of which, nevertheless, seem to be somewhat thicker 

 than those of the auricle. A circular constriction marks the union of the two 

 chambers, and two valves guard the auriculo-ventricular opening. The base of 

 the aorta is slightly dilated and highly contractile, being furnished with valves 

 at its origin from the ventricle. It very soon gives rise to a large vessel which 

 passes downwards and backwards to supply the viscera; but the great podo- 

 cephalic division (Plate I. fig. 4, b) crosses over the alimentary tube from the left 

 to the right side, and, having formed a few flexures, runs forwards alongside the 

 oesophagus until it arrives a little in front of the pedal ganglia, where it curves 

 downwards to give rise to the pedal artery (Plate I. fig. 3, c), which opens directly 

 into the great ventral sinus at the root of the swimming-plate. From the origin 

 of the latter vessel the main trunk courses forwards as the buccal artery, and, 

 having reached the fundus of the tongue-sac, it is no further traceable. 



Generatim System. — The external male organ lies on the right side of the 

 body near its posterior extremity, and consists of two portions, one of which is 

 of considerable length, terminating in a bulbous enlargement (Plate I. fig. 1, m), 

 but the other is short, with the orifice or terminal part usually inverted. The 

 follicles of the testicle converge to a duct, which exhibits a fusiform, though 

 twisted dilatation (Plate I. fig. 1, I) in its course, and which is, moreover, dis- 

 tinguished by its coating of black pigment cells ; but the duct does not appear to 

 reach the external organ, or, if it does in this case, nothing of the kind is dis- 

 coverable in the other genera of Heteropoda. 



The female orifice (Plate 1. fig. 4, i) is situated immediately above the root of 

 the tail, and guarded on each side by a small laterally compressed leaf-like process, 

 somewhat larger than the anal lobes previously noticed. The oviduct (Plate I. 

 fig. 4, 1) is rather capacious, and once or twice doubled upon itself The ovarian 

 sacculi communicate with its inner extremity; and the impregnated ova, with 

 which the duct is sometimes found distended, may be observed in all stages of 

 yelk- cleavage, exhibiting further advancement as they approach the external 

 opening, from which a delicate nidamental chord with ova, in single or two al- 

 ternating series, may be frequently seen protruding (Plate I. fig. 6), 



Atlanta (Lesueur). 



As the remarks which I have to make on the anatomy of Atlanta have wide 

 reference to all the species of the genus, I shall defer the perplexing question of 

 specific determination to a future paper. 



General Sketch of the Genus (Plate II. Fig. 1). 

 The shell of Atlanta is dextrally-spiral in the young state; but it subse- 



VOL. XXIII. PART I. c 



