168 HUGH WATSON. 



The slieath of tlie odontopliore is composed of three layers 

 of muscle-fibres. Externally there is a thin layer of longi- 

 tudinal muscles; within this lies a layer of circular muscles, 

 which is equally thin throughout the greater part of the 

 length of the odontophore, but becomes a little thicker 

 toAvards the front end ; lastly, there is a very thin layer of 

 longitudinal muscles lining the whole of the sheath excepting 

 the dorsal part which lies between the attachment on each 

 side of the suspensor muscles of the support. At its hind 

 end the sheath of the odontophore becomes intimately united 

 with the support and the posterior ends of the retractor 

 muscles, but throughout nearly the whole of its length its 

 only connection with the underlying tissues is by means of 

 the suspensor muscles. 



I have examined the arrangement of the muscles of the 

 odontophore in all the species of Apera excepting A. 

 purcelli, and the amount of variation is so small, notwith- 

 standing the diversity of the radulae, that it must be admitted 

 that these muscles are of considerable systematic importance. 

 It will, therefore, be of special interest to compare the 

 arrangement of the odontophoral muscles of Apera with 

 that which is found in other families of carnivorous snails 

 and slugs. 



As an example of the Rhytidid^e we may choose 

 Natalina. Nineteen years ago M. F. Woodward^ published 

 an account of the anatomy of Natalina caffra {Fer.), Avith 

 special reference to the structure of the odontophore ; but 

 unfortunately his account shows evidence of inaccuracy. I 

 shall therefore ignore his description and figures, and compare 

 the odontophoral muscles of Apera with those of a species 

 of Natalina, nearly allied to N. caffra, which I have 

 been able to examine myself, namely N. quekettiana {M. & 

 P.). On the whole the odontophore of this species resembles 

 that of Apera very closely; it differs, however, in the 

 following features. — (1) Instead of being straight the odonto- 

 phore is slightly curved towards the left ; moreover its hind 

 ' ' Proc. Mai. Soc.,' 1895, vol. i, pp. 270-277, pi. xvii. 



