nS HUGH WATSUN. 



5 inm. in front of the opening (PI. VII, figs. 1, 4 ; PI. VIII, 

 figs. 22, 24). 



A single median posterior groove runs backwards from 

 the orifice. It is usually deep and conspicuous in front, but 

 breaks up into finer irregular grooves before reaching the 

 hind end of the slug. 



Well-marked radial grooves diverge laterally from the 

 region of the respiratory opening. About four of these on 

 each side arise from the opening itself, and two or three others 

 branch from the median posterior groove behind the opening. 

 In Apera purcelli,the most anterior grooves, instead of 

 arising from the respiratory opening, branch from the dorsal 

 grooves in front of it (PL VIII, fig. 25). As a rule the radial 

 grooves extend down the sides of the body, but in A. gib- 

 bon si some of tliem usually bifurcate before reaching the 

 edge of the foot. The most anterior grooves of A. gibbonsi, 

 A. parva, and A. dimidia always divide within a short 

 distance of the respiratory opening, the lower branch sloping 

 downwards towards the foot, while the other continues forward. 

 The latter branch soon divides again in a similar manner, 

 and this process is repeated several times, so that an irregular 

 lateral groove is formed, which runs forward parallel to 

 the dorsal groove and gives off oblique sub-lateral grooves 

 parallel to one another. The lateral grooves are seldom well 

 developed in A. gibbonsi and A. dimidia, but the oblique 

 sub-lateral grooves are rather conspicuous in the former 

 species (PL VII, figs. 2, 3, 5, 6). On the other hand, in A. 

 parva, A. burnupi, and A. sexangula the lateral grooves 

 are more definite and better developed than the sub-lateral 

 grooves, of which there are only a snui.ll number (PL VIII, 

 fig. 2:3; PI VII, figs. 11, 13). The latei-al grooves of the two 

 latter forms are lower down than those of the other species, 

 and branch from the most anterior pair of radial grooves at 

 a greater distance from the respiratory opening. They run 

 along between the upper and lower keels as far as the head, 

 the right lateral groove ending in the genital ojiening as in 

 Testae el la and other forms. It is probable that the greater 



