164 HUGH WATSON. 



less tinged witli brown, as is so commonly the case among 

 the carnivorous genera. 



I^HE Odontophore. — The hind end of the buccal mass is 

 prolonged beyond the opening of the oesophagus to form the 

 cylindrical muscular odontophore containing the radula-sac. 

 In most carnivorous snails the hind end of the odontophore is 

 curved abruptly downwards, but in Apera, as in Testacella, 

 it has become quite straight, probably owing to the degenera- 

 tion of the spiral shell. The size of the odontophore — and, 

 to a less extent, the size of the buccal mass in front of it — is 

 proportionate to the size of the radula. In A. burn up i 

 (PL XXI, fig. 133) and A. sexangula (fig. 134) it is 

 quite small, being onl}^ about 4 mm. long; in A. purcelli 

 (PI. XIII, fig. 55) and A. dimidia (Pl.'xXI, fig. 132) it is 

 much larger; finally, in A. gibbonsi and A. parva 

 (figs. 129-131) the odontophore attains an enormous size, 

 being one of the largest organs in the body. Yet the 

 structure of the odontophore is remarkably constant through- 

 out the genus, the differences between the arrangement of its 

 muscles in the various species being quite insignificant. 



The odontophoral support or cai-tilage is a semi- 

 cylindrical structure which extends along almost the entire 

 length of the odontophore. It is crescentic in section, the 

 convex surface being ventral. In the hollow formed by its 

 upper surface lie the radiila-sac and the muscles immediately 

 surrounding it, while the ventral pocket containing the front 

 end of the radula is situated beneath the anterior part of the 

 support, the radula being folded over its anterior end (PI. 

 XVII, fig. 92, and PL XVIII, figs. 115-122). The upper edges 

 of the support are parallel throughout the greater part of its 

 length, but anteriorly they curve inwards towards each other, 

 and then diverge again and slope downwards at the front 

 end, thus leaving an oblique oval aperture where the opening 

 of the radula-sac is situated (PL XVII, figs. 95, 100, 105, 112). 

 As might have been expected, this aperture is largest in 

 Apera gibbonsi, and the front end of the support is 

 rounded in this species and in A. parva. On the other 



