14 HiCKLING, Variation of Planorbis untltiforinis, Bronn. 



indication that the shells were the descendants of several 

 distinct ancestral types, which had become similar by the 

 acquisition of similar modifications. Hyatt did, in fact, 

 to a limited degree, draw this conclusion, though the 

 phenomenon of homceomorphy was not adequately under- 

 stood at the time he wrote. Nevertheless, the explanation 

 is not applicable to the present case, for, as a reference to 

 PI. II., Fig. 4, where a series of young forms is photo- 

 graphed, will show, all the types of young pass insensibly 

 into one another. In other words, the young stages 

 represent, not several distinct species, but a single highly 

 variable ancestral form. 



The actual variations of the young shells appear to 

 be entirely due to the development, in different degrees, 

 of carination. This may be completely absent {PI. 11, 

 Fig. 4 a), giving the " roiiifidaliis-Wke. young " of Hyatt. 

 In others the median carina appears alone, and finally 

 both carina; may be present. As the two carinae become 

 more strongly marked, the section of the whorl becomes 

 correspondingly more rectangular. There can be little 

 doubt, either on general grounds or from a consideration 

 of the actual geological history of these shells as recorded 

 by Hyatt, that the smooth form is the ancestral one. 

 Further, the fact that all stages of carination appear in 

 young shells which are yet completely discoidal is a 

 sufficient proof that in phylogeny carination was acquired 

 before turreting. 



General Conclusions and Discussion. 



The main conclusions here reached with regard to 

 Planorbis nuiltifonnis are : — 



(i) The ancestral 'form of the shell was discoid and 

 smooth, with whorls of rounded section. 



