160 HUGH WATSON. 



making it more difficult for the teeth to be pulled out of posi- 

 tion and keeping- them rigidly in place. Lastly, the teeth 

 become modified in one of two ways in order to prevent the 

 worm that they have transfixed from escaping. In A. gib- 

 bonsi rubella the cusps are strongly curved so as to form 

 veritable hooks (text-fig. 3, b), and they are also slightly 

 broadened towards the points. On the other hand, in A. 

 gibbon si lupata the cusps are bai'bed on the lower side, as 

 in Testacella — a remarkable case of parallel evolution (cf. 

 PL XXIV, fig. 157, and text-fig. 3, d). Still more highly 

 specialised is the var. duplex of the same form. In this 

 variety all the larger teeth are doubly barbed, there being a 

 small barb on the upper side of the cusp nearer to the point 

 than that on the lower side (text-fig. 3, e). I do not know 

 of any other Pulmonate in which the radula has become so 

 highly specialised along these lines as it has in this variety. 

 We have to go to the marine carnivorous genera, such as 

 Conus, to find such formidable doubly barbed teeth, and here 

 also we notice that one barb is nearer the point of the tooth 

 than the other. In Conus, however, the barbs are pointed 

 instead of being merely square as in Apera, and the distal 

 barb appears to have been developed first, for the other is 

 often absent or merely represented by a flange. And of 

 course the bases and arrangement of the teeth in Conus are 

 quite different. 



The radula of Apera parva bears a general resemblance 

 to that of A. gibbonsi, but the bases of the teeth are not 

 so narrow, the cusps of the inner teeth are slightly shorter, 

 and the teeth become relatively larger towards the edges of 

 the radula (text-fig. 4, a). None of the transverse rows 

 contain more than thirty-five teeth in this species, and some 

 have only thirty-four, as the vestigial central tooth is absent 

 from some of the rows. On the Avliole the radula of A. 

 parva is less unlike the type found in the genus Rhytida 

 than are the radula of the other species of Apera. 



In Apera purcelli and A. dimidia the central tooth 

 also degenerates. It is present, though small, in A. purcelli 



