THE RULES OF NOMENCLATURE 367 



Cardita deltoidea J. Sowerby = Pholadomya deltoidea 

 (J. Sowerby). 



Venericardia deltoidea ]. Sowerby = Cardita deltoidea 

 (J. Sowerby). 



A wise precaution to avoid such possibilities of con- 

 fusion is never to use the same trivial name in genera at 

 all nearly related. 



5. A later author may divide a genus into several 

 genera. The original name must then be retained for 

 one of these divisions, and, if possible, for that division 

 which was regarded by the author of the original genus 

 as most typical. With a view to this possibility the 

 founder of a genus should always name one species as 

 most typical : such a species is called the type-species of 

 the genus, or, more shortly, the genotype (see later). The 

 absence of a specified genotype from the original defini- 

 tion of a genus has often been a source of much con- 

 fusion when the genus has been subdivided later. 



The subdivision of a genus often takes place in two 

 stages : one author divides it into sub-genera^ and a later 

 author raises these to the rank of genera. To provide for 

 this possibility it is a rule that when a genus is divided 

 into sub-genera, one of these (the one including the 

 genotype) must bear the same name as the genus itself. 

 To distinguish the sub-generic name from the generic 

 name, the former is written with the addition of the 

 abbreviation s. -str. (sensu stricto, in the restricted sense). 

 Thus Hall and Clarke divided the old genus Orthis of 

 Dalman into a number of sub-genera, to all of which 

 except one new names were given, such as Dalmanella, 

 Schizophoria, etc., but the one which included Dalman's 

 genotype was called Orthis s. str. Under this scheme the 

 specific names of some well-known species became 



Orthis (Dalmanella} elegantula Dalrnan ; 

 Orthis (Schizophoria) resupinata (Martin) ; 

 Ovthis (Orthis) callactis Dalman. 



