112 PROCEEDINGS OE THE MALACOLOGICAE SOCIETY. 



It will be seen from the description of the anatomy of Acavus, 

 that this genus presents us with a very compact group, no one species 

 of which differs in any marked respect from any other species. Still, 

 sufficient differences exist, especially as regards the genitalia, to 

 enable us to accurately determine the species. The genus is divisible 

 into two sub-groups, represented by the types A. hcemastomus and 

 A. Waltoni respectively, the conchological differences of which are 

 more marked than the anatomical. These two sub-groups differ also 

 with respect to their habitat, the former being entirely arboreal, 

 whilst the latter forms are terrestrial, living amongst fallen logs and 

 dead leaves. 



The species A. Poleii, founded by Collett, has been regarded as 

 merely a variety of A. Waltoni by other conchologists, but the two 

 forms are quite distinct, and besides differing in the colour of the 

 animal, and the habitat, also present anatomical differences in the 

 structure of the penis, which can scarcely be regarded as less than 

 specific. 



Unfortunately this paper cannot be regarded as complete, since 

 there are two so-called species of Acavus of which I have been unable to 

 obtain specimens, viz., A. prosper us (Albers) and A. fastosus (Albers). 

 In addition there are two varieties of A. hcemastomus which were 

 not included amongst the specimens sent for examination. These are 

 A. hcemastomus, var. conns, Pilsbry, and var. concolor, Pilsbry. It is 

 extremely unlikely that either of the above species differ in any 

 considerable respect from those of the A. hcemastomus group, and it is 

 probable that they are more nearly related to A. hcemastomus than any 

 other form. 



EXPLANATION OF PLATE IX. 



Fig. 1. General dissection of Acavus phoenix:. 



,, 2. Transverse row of teeth from the radula of Acavus hcemastomus. c. median 

 tooth; 1, 3, 6-40, etc., are lateral and marginal teeth. The corre- 

 sponding teeth of the other species have been figured in the same order, 

 except where otherwise indicated by the number of the tooth. 



,, 3. Radula of Acavus superbus. 



,, 4. ,, ,, phoenix. 



,, 5. ,, ,, Skinneri. 



,, 6. ,, ,, Waltoni. 



,, 7. ,, ,, Poleii. 



„ 8. Diagram representing the hermaphrodite caecum and associated structures, 

 and showing its connection with the hermaphrodite duct. Reconstructed 

 from serial transverse sections. 



,, 9. Transverse section through the hermaphrodite caecum of Acavus hcemastomus. 



,, 10. Genitalia of Acavus Waltoni. Anterior part. 



,,11. _ „ ,, Poleii. ,, 



,, 12. Penis of Acavus hcemastomus. A longitudinal incision has been made 

 along the mid-dorsal line, and the two halves have been reflected. 



,, 13. Penis of Acavus phoenix. 



,,14. ,, ,, superbus. 



„ 1-5. ,, ,, Waltoni. 



,, 16. ,, ,, Poleii. 



