Manchester Memoirs, Vol. ivii. ( 191 3), A^^. 10- 15 



(2) Carination was subsequently developed, producing 

 a complete range of discoid types, varying from the most 

 strongly carinate, with whorls of rectangular section, to 

 the ancestral non-carinate form. 



(3) Later variation gave rise to turreted forms, this 

 character being also developed in every degree, and 

 arising in all the pre-existing varieties. 



(4) Throughout all these changes, the whole group of 

 shells has maintained complete biological unity, as shown 

 by the fact that the co-existing ultimate forms here 

 described have all their varieties connected by a typical 

 "continuous variation" curve, and must therefore be 

 regarded as constituting a single species. 



The simplest conception of the species as here de- 

 scribed may be expressed by defining it as a Plancrbis 

 which has become potentially carinate and turreted, either 

 of which potential characteristics may be inherited by the 

 individual in any degree, the frequency of the inheritance 

 following the law of error. 



The minor variations described above may all be 

 regarded as essentially consequences of these two prin- 

 cipal modifications, separately or in combination, and, for 

 the sake of clearness, may be neglected in a general 

 discussion. With this restriction it is evident that four 

 types of Planorhis inultiformis potentially exist : — 

 (i) Non-carinate discoid. 



(2) Carinate discoid. 



(3) Non-carinate turreted. 



(4) Carinate turreted. 



Further, it has been shown that carination is now in a 

 declining condition, since individuals which are carinate 

 in an early stage may become smooth on further growth. 

 Hence it follows that two other types, v/hich may be 



