r.WATKIX : POME MOLI.USCAN RADUL.E. I47 



— Seventeen species, fairly universal, if the want of Australian species 

 is accidental. They are mostly large species. 



Our fourth section of A/uyliis appears to be limited to the Coosa 

 region of Alabama, which is also the home of the Sfrepoiiuxlidie. Of 

 this I have seen four species : — 



A. elatior ^V'alker. A. gwatkinianus Walker. 



A. filosiis Conr. A. rhodacme Walker. 



This, again, seems a development of the Southern type, and a very 

 remarka!)le development. Tiie general contour of the radula actu- 

 ally reminds us of Brachypodella, and there is a real likeness. In 

 general the teeth are of the southern type; but the central is small 

 and unicuspid or faintly bicuspid, the six or eight laterals very large, 

 in angulated rows, with the angles pointing backwards — the rows on 

 each side are straight, with the cusps rapidly vanishing. The jaw is 

 of many plates, but they do not seem so separate as in A. fluviatilis: 

 So far there is nothing very unlike things we have seen before ; but 

 now comes a great anomaly. Each lateral has a second cusp, a 

 large oval tooth with a simple broad cutting edge; and these acces- 

 sory cusps grow smaller as we pass away from the central tooth, 

 vanishing about where the laterals pass into marginals. As in 

 Brachypodella, the second cusp is not beside the chief one, but 

 rises from the hinder end of the base. 



Putting all this together, the oldest type of radula we have seen 

 would seem lo be that of the southern section oi Ancylus — Type (A), 

 and the weak species of Planorhis. Where it originated is best left 

 uncertain, for it may date back long before climatic and other condi- 

 tions gave Northern Europe its present dominant position. From this 

 would diverge in distant ages the strong species oi Planorhis. Isidora, 

 and Tvpp: (B) of Ancylus differ little from these. It is remarkable 

 that Isidora does not reach America, though it is found as far north 

 as Algeria. 



If Physa (including Aplecttx) is derived from the same stock, it has 

 undergone a strange development. The radula is shorter and differ- 

 ently shaped, the rows are angulated, the central tooth is peculiar — • 

 perhaps a modification of a primitive bicuspid — the laterals have 

 disappeared, and the whole radula is an expanse of comb-like mar- 

 ginals with a basal process. The central tooth and this basal process 

 are the chief ditificulties, for I have noticed nothing like them else- 

 where. The distribution also of Physa is anomalous. Like the 

 higher IMammalia, it has not reached Australia and Polynesia— at 

 least all the Physas I have seen from those parts have the radula of 

 Isidora (including P/iysopsis). Nevertheless, there is a true Physa 

 (Physa coin^ada Old.) from the Hawaiian Islands. Either we have 



