162 HUGH WATSON. 



(text-fig. 4, c), but no trace of it remains in A. dimidia 

 (PI. XX, fig. 127, and text-fig. 4, b). The outer teeth of 

 these two species have rather long, slightly curved cusps, and 

 are not very unlike the teeth of the typical form of A. gib- 

 bon si on a smaller scale; but the cusps of the inner teeth 

 are remarkably short and broad, and the teeth have therefore 

 a quite different appearance. In the more primitive species, 

 A. purcelli, only the first three or four teeth on each side are 

 thus modified, but in A. dimidia half the teeth are of this 

 shape. Hence the teeth of these species, unlike those of nearly 

 all other carnivorous forms, are differentiated into laterals and 

 marginals, although there are one or two on each side which 

 might be regarded as transitional. The figures of the radula 

 of A. dimidia (PI. XX, fig. 127, and text-fig. 4, b) show that 

 in this species the corresponding teeth on each side are not 

 opposite to each other, the right half of the row being in front 

 of the left. A similar displacement occurs in A. parva 

 (text-fig. 4, a), and it is also often observable in A . gibbon si 

 (PI, XX, fig. 125) ; but it occurs occasionally in other carni- 

 vorous genera, for I have noticed it in Test ace 11a halio- 

 ti dea Drap., T. scutulum Soiv., Euglandina truncata 

 {Gmel.), E. corneola {Binn.), and Rhytida franklandi- 

 ensis {Forhes). 



In Apera sexangula and A. burnupi the difi^erentia- 

 tion into lateral and marginal teeth is still more marked (PI. 

 XX, fig. 128 ; text-fig. 4, d, e). The marginal teeth are more 

 than twice as numerous as the laterals, and their cusps are 

 long, slender, and nearly straight, those of one row over- 

 lapping the bases of the teeth in the row behind. The 

 lateral and central teeth have short broad cusps, and the 

 central tooth — which is only slightly smaller than those on 

 each side of it — is not unlike the lateral teeth of A. purcelli 

 and A. dimidia, though perhaps a little shorter. The cusps 

 of the lateral teeth, however, are not only very short, but 

 they are bifid and end in two points, the inner of which is the 

 longer (text-fig. 4, d-f). Beutler^ has shoAvn that in Pary- 

 » ' Zool. Jahrb.; 1901, vol. xiv, p. 380, pi. xxvii, fig. 23a. 



