764 PEOF. G. C. BOURNE ON THE 



transparent, the musculature is feeble, and the epidermis is not 

 pigmented. This dorsal region is relatively of considerable length ; 

 its concave lower border corresponds very closely in length with 

 the surface of insertion of the left columellar muscle ; its upper 

 surface extends back to the pericai-dium. It contains nearly all 

 the coils of the intestine, the esophagus, the radular sac, and the 

 hinder lobes of the cesophageal pouches. Its roof forms the floor 

 of the hinder part of the mantle-cavity. In consequence of the 

 elongation of these two regions, but particularly of the dorsal 

 region, the mantle-cavity is continued very far back ; so far that, 

 measured from its most anterior to its most posterior limit, it 

 makes nearly a complete turn of a spiral, whereas in N'erita and 

 Neritina it makes little more than half a turn. Broad in front 

 where its roof passes from the right to the left columellar muscle, 

 the mantle-cavity becomes narrower- and narrower posteriorly and 

 ends in a pointed cul-de-sac below and somewhat to the right of the 

 lower surface of the visceral mass. Its extreme posterior limit is 

 not quite visible in fig. 1 . With the hinder end of the mantle- 

 cavity the pericardium has also been cari-ied very far back. It is 

 laid open in fig. 1 to show the position of the heart. It will be 

 seen that the single auricle seems to lie behind the ventricle, and 

 not in front of it as does the larger left auricle in the Neritidse. In 

 horizontal sections, such as those depicted on PI. XXXIII. figs. 18 

 and 19, this posterior position of the auricle forces itself so much 

 upon one's attention that I was led to form the theory that the 

 single auricle of the Helicinidte corresponds not to the left and 

 laro-er, but to the right and rudimentary auricle of the ISTeritidse ; 

 and in my memoir on the morphology of the latter family (2, 

 p. 833) I prematurely gave expression to this view, which seemed 

 to me the more pi^obable because I found that in correlation with 

 an increase of the pallia! vessels in Septaria the right auricle 

 became relatively larger and took an obviously larger share in 

 carrying blood from the pallial vessels to the ventricle. But since 

 then, after a careful study of the relative positions of the kidney, 

 the uropore, the rectum, and the heart in the Helicinidse, I have 

 satisfied myself that this view was erroneous. 



As a consequence of the dorsal elongation of the body all 

 these organs have been rotated through an angle of rather more 

 than 90°, in such wise that the pyloric end of the stomach, which 

 in the Neritida; is directed forward and to the left, comes to lie 

 at the right posterior end of the body in the Helicinida?, and 

 the swollen oesophageal end of the stomach, which is posterior 

 and somewhat to the left in the Neritidae, is directed anteriorly 

 and to the right in the Helicinidae, forming a conspicuous 

 rounded prominence at the extreme end of the visceral mass 

 (PI. XXX. fig. 1, St:). To understand the nature of this rotatory 

 movement the reader should refer to fig. 42 of my memoir on 

 the ISTei-itidge. This figure represents a horizontal section of 

 Paranerita gagates, and shows the position of the oesophageal (St.) 

 and pyloric {St.') divisions of the stomach. It should be noticed 



