HANDLES: ANATOMY OF ACAVTTS. 107 



teeth in the row. Semper l enumerates the number of teeth iu 

 a transverse row of the radula of A. lwmastomus from 97 to 101 ; in 



1 the specimens that I examined the number varied from 87 to 101. 



! Whether this expresses the extreme range of variation or not, can 

 only be determined after the examination of a greater number of 

 specimens than I had at my disposal. In two specimens of 



" A. hcemastomus, var. melanotragus, that I examined, the number 

 of teeth in a row was constant, viz. 97. 



The radulse of A. superb us (Fig. 3) and A. phoenix (Fig. 4) are 



L stouter and broader than that of A. hamastomus, and the individual 

 teeth, though very similar in contour, are larger. The marginal 



' projection on the lateral teeth is more marked in these species, and 



* in A. phoenix attains its maximum development. 



In A. superbus the number of teeth in a transverse row varies from 

 101 to 105, the type form possessing 101 teeth, the variety ro&eolabiata 

 103, and the variety Grevillei 105. 



A. phoenix possesses from 105 to 117 teeth in a row. Binucy has 

 figured the teeth of this species, but his drawing is somewhat 

 inaccurate, and was probably made from an old and worn part of the 

 radula. 



Taking the three aforementioned species as representative of that 

 sub-group of the genus Acacus of which A. hcemastomus is the type, 



1 it would appear that as regards the radula, there is a gradual transition 

 of the one species into the other, and that there is a tendency to an 

 increase in the number of teeth in a transverse row, reaching its 

 maximum in A. phoenix, but nevertheless foreshadowed in the varieties 

 of A. hcemastomus. Coming to that sub-group of Acavus of which 

 A. Waltoni is the type (the subgenus . OUgospira of Ancey 2 ), we find 

 that a similar variation in the number of teeth in a transverse row 

 occurs. The minimum number of teeth present occurs in A. Shinneri 

 (Fig. 5), 73 to 79. The two specimens of A. Waltoni (Fig. 6) 

 examined possessed 103 teeth in each row. The maximum number 

 was reached in A. Poleii (Fig. 7), in one specimen of which as 

 many as 123 teeth were present in a row, though iu another only 

 103 teeth occurred. 

 These results can be tabulated, and an average dental formula 



' assigned to each of the species of Acacus^ as follows : — 



\ „ . Number of Limits of Average radula 



species. specimens examined. variation. formula. 



A. haemastomus 9 ... .87-101" ... 47-1-47 



A. superbus 4 ... 101-105 ... 51-1-51 



A. phoenix 5. ... 105-117 ... 56-1 55 



A. Skinneri 'I ... 71-77 ... 36-1-36 



A. Waltoni 2 ... 103 ... 51-1-51 



A. Poleii 3 ... 103-123 ... 56-1- 



a; 



The pulmonary sac is large ; the collar is very much thickened, and 

 is provided with two well-marked body lobes (PI. IX, Fig. 1, b.l.) ; 



1 Op. cit., p. 100. 



2 Conchologists' Exchange, vol. ii (1887), p. 22. 



