218 HUGH WATSON. 



sexangula in not a few characters, notably the pedal gland, 

 the long penial retractor, and the complete separation of the 

 dorsal grooves. Moreover, it is probable that the ancestral 

 form from which A. burnupi and A. sexangula have been 

 evolved resembled A. purcelli still more closely, since the 

 keels and the double cusps of the lateral teeth of these species 

 cannot be regarded as primitive characters. Yet it is not 

 likely that in A. purcelli we have the direct ancestor of 

 these two species, for A. purcelli has in some respects 

 become modified in a different direction, its central tooth 

 having become smaller and the course of the intestine having 

 become quite different from what we must regard as the 

 more primitive arrangement found in A. burnupi, A. sex- 

 angula, and A. gibbonsi. It is therefore probable that 

 A. purcelli must be regarded as having diverged somewhat 

 from the branch which gave rise to the keeled species. 



Apera dimidia in some respects resembles A. purcelli, 

 but it is much more highly specialised. The glandular tissue 

 of the pedal gland has become entirely concentrated around 

 the anterior half of the duct ; the central tooth of the radula 

 has disappeared, and the laterals have become more numerous ; 

 the penial retractor has become quite short ; the dorsal 

 grooves are united posteriorly: in fact, while A. dimidia 

 has probably been derived from the same branch as A. 

 purcelli, it has diverged very far from both A. purcelli 

 and the keeled species. 



Apera gibbonsi and A. parva differ widely from the 

 other species in their enormous radula and numerous radial 

 buccal retractors, as well as in the oesophagus, receptacular 

 duct, shell, coloration, etc., and apparently in the structure 

 of the terminal vesicle of the pedal gland. In some respects 

 they resemble A. dimidia most closely, especially in the 

 absence of glandular tissue from the posterior end of the 

 pedal gland, the short common duct, the union of the dorsal 

 grooves in front of the respiratory opening, and the position 

 of the lateral grooves ; and it might be possible to regard 

 A. gibbonsi and A. parva as being derived from the 



