134 Annals of the South African Museum. 



And now, having taken as it were a bird's-eye view of the 

 Acavidae as a whole, let us examine more closely those members 

 of the family that live in South xVfrica. 



Sub-Family DOECASIINAE, nov., 1915. 



Shell rather large, perforate, elongate-globose to depressed- 

 orbicular ; fairly solid ; usually almost unicoloured, with little 

 ornamentation, transversely striate or costate, frequently showing 

 nialleation, but little or no spiral sculpture. Aperture large and 

 toothless (except in Tulhaghinia), margin invariably thickened or 

 reflexed. 



External features of Animal. — Foot broad and rather short, more 

 or less rounded at the hind end. Sole rather obscurely tripartite in 

 Dorcasia, but undivided in Trigoneplirus ; covered by a ciUated 

 columnar epithelium, and apparently without unicellular glands. 

 Eest of skin divided into polygonal rugae. There are no well- 

 marked peripodial, dorsal, or median posterior grooves, but a lateral 

 groove rvms down on each side from the mantle-edge to the head. 

 The right lateral groove ends in the genital opening, which is almost 

 as far forward as the upper tentacles, but lies nearer the foot. Beneath 

 the lower tentacles there is a pair of broad and conspicuous labial 

 lobes. No caudal mucous pore is present. 



Mantle-edge thick, usually with right and left body-lobes. The 

 left lobe is often divided into two widely separated portions, one 

 near the respiratory opening, the other on the left side of the animal ; 

 but in most cases the two portions are united by a fold (see text- 

 fig. 2, A, and PI. IV., figs. 3-6). There are no shell-lobes. 



Respiratory system. — Mantle-cavity or lung rather short. Pul- 

 monary veins branching over the whole of its roof ; usually rather 

 more numerous between the respiratory opening and the peri- 

 cardium and kidney than elsewhere, but never forming a very dense 

 network. The first branch of the pericardial vein is almost as large 

 as the main pulmonary vein, and runs forward nearly parallel to it, 

 a large afferent vein lying between them. Beyond the first branch, 

 the pericardial vein gives off one or two smaller branches, which 

 alternate with afferent veins coming from the mantle-edge. Small 

 efferent and afferent veins also cross the space between the rectum 

 and the kidney and the main pulmonary vein (see text-fig. 2, A, 

 and PI. IV., figs. 1-6). 



Heart and pericardium very oblique; the auricle lying nearer to the 



