56 



viduals, but is so within certain limits for the same species. 

 The central tooth or plate is symmetrical and the lateral diverge 

 from it in form as they are distant from it." They do this 

 according to a certain rule, which is, " By the smrpression of 

 the prominences on the inner side of each lateral tubercle, and 

 by the increase of the corresponding parts on the outer side." 

 Again, "a gradual curve in the transverse line causes a gradual 

 change in the form of the teeth ; great angularity causes a 

 sudden change : and the degree of duration from a straight 

 line is the measure of the difference between the central and 

 lateral teeth." 



These valuable observations give the key to the form of the 

 teeth in Siphonaria, though subject to considerable modifica- 

 tions. In a figure after Wilton in Woodward {loc. cit. p. 305) 

 the teeth of a Cape Siphonaria is given (8. venosa ?') where 

 the teeth are not in plates but linear, hooked, the flate or 

 tubercle being both oblique, but the plate sloping from the 

 centre, and the hooked tubercle towards it, with about 40 on 

 each side, the transverse line curved upwards. 



In H. denticulata, the buccal mass is red and fleshy, in which 

 two long, thin, rather broad cartilaginous jaws are imbedded. 

 Amid these the broad ribbon is spread, working almost per- 

 pendicularly, with a very slight movement backwards, as far a& 

 I could ascertain in the few opportunities which the shy and 

 sluggish animal gave me of observing. The oesophagus is a 

 bright orange yellow, and terminates at the distance of about 

 20 mil, in a sac of the same colour. The odontophore soon 

 becomes a tube enclosed in membrane. It does not follow 

 the oesophagus, but curls round and projects as a closed 

 hyaline tube out&ide the buccal mass. When the animal is 

 wounded it admits a viscid milky blood (?) of quite a different 

 character from other gasteropods. 



The odontophore with careful manipulation may be easily 

 extracted and spread out. It is very difficult to clean it from 

 the attached membranes, but when spread is about 8 mil. long 

 by 3 broad. It is a series of curved lines of teeth diminishing 

 in size from the centre to the margin. The teeth have a broad 

 crescentic edge, which increases in width downwards and is fixed 

 upon the membrane. The teeth gradually diminish out- 

 wardly to a mere faint line of curved tubercles. The appear- 

 ance of the whole is more like a series of combs with long 

 curved teeth. There appears to be, properly speaking, no 

 plate from which each tooth projects, and the central tooth 

 from which each row diverges in a curved line, is rudi- 

 mentary. , 



Siphonaria diemenensis. Quoy and Gaim. Loc. cit., 

 vol. 2, p. 327, pi. 25., fig. 1 to 12. Shell oval, convex some- 

 what high, apex acute, median, with numerous small, con- 



