AND CLASSIFICATION OF THE HETEROPODA. 13 



backwards, gradually increasing in breadth, and finally forms the floor of the 

 tubular tongue-sac. 



The rachidian plates (Plate 11. fig. 5, g, fig. 8, o) are quadrilateral, and thicker 

 posteriorly than in front, so that the corresponding angles are much more clearly 

 defined. Each plate bears a single sharp conical tooth, arising directly from the 

 middle of the posterior border. 



The anterior and internal part of the first series of pleural plates (Plate II. fig. 

 8, 1.) abuts upon the members of the rachis, and there is never a small tooth on the 

 inner side of the large terminal fang, though there is very generally a characteristic 

 minute spine-like tooth (Plate II. fig. 8, i") a little to the outer side of its base. 



The two outer series of the pleurae (Plate II. fig. 8, 2, 3 ), as in all the other 

 Heteropods, consist of simple tenaculiform hooks, varying in relative length, 

 strength, and curvature, in the different species of the genus. 



The configuration of the teeth, and the mode of action of the whole dental 

 apparatus, are adapted for seizing and transfixing living prey; indeed, all the 

 Heteropods are eminently carnivorous, and commonly bolt their victims whole. 

 I have taken a Firola, which I recognised by its dentition, from the stomach of 

 Carinaria. I also found a whole Eurybia impacted in the gullet of another speci- 

 men ; and it is not an uncommon thing to meet with one of its own kind in the 

 stomach oi Atlanta, although it more usually preys upon the smaller Pteropods, 

 Spiralis, or Creseis, for example. 



From the so-called buccal mass (Plate II. fig. 1, h) a lengthy oesophagus courses 

 directly backwards, and having entered the base of the visceral chamber, it soon 

 opens into the anterior and inferior wall of a simple sub-globular stomach (Plate II. 

 fig. 1, 7). This latter viscus lies a little below, and posterior to the heart (Plate 

 II. fig. 1, y), and from its upper and fore part, a short intestine (Plate II. fig. 1, x) 

 passes forwards above, and nearly parallel with, the oesophagus, and in close relation 

 to the heart, to end in the vent (Plate II. fig. 1, s), deeply within the mantle cavity. 



The liver (Plate II. fig. 1, e)is a beautifully sacculated gland, of a pale-brown 

 tint, often mottled with amber, black and red, commencing by a small pointed ex- 

 tremity near the nucleus of the shell, and gradually increasing in bulk, until it 

 reaches near the posterior wall of the stomach, into which a single short and 

 stout biliary duct opens. 



I may notice here a conspicuous glandular body, which appears to me to be a 

 renal organ (Plate II. fig. 1, u). This lies immediately above the rectum, and 

 between the latter and the auricle of the heart, and is made up of densely clustered, 

 minute, and rounded follicles, communicating freely with each other ; but I can- 

 not tell whether its very distinct oval outlet (u) opens into the heart, pericardium, 

 or into the cavity of the mantle. Between the apparent position of this orifice 

 and the arms there is a small prominence, bearing a well-defined pore (Plate 11. 

 fig. 1, t) that may communicate with it, or with an aquiferous system. 



VOL. XXIII. PART I. D . 



