THE NAUTILUS. 89 



The evidence offered for either view may be condensed as follows: 



The original genus Uin'o of Retzius, 1788, contains species both 

 with and without lateral hinge teeth, and no type is named. In 

 1792 Bruguiere describes the genus Unio as possessing such teeth, 

 without saying, however, what is to become of those species which 

 have no lateral teeth. In 1815 Oken divided the genus Unio, call- 

 ing the species with lateral teeth Lymnium and those without teeth 

 Unio. In 1817 Schumacher did the same, but retained Unio for 

 the species with teeth, while for those without teeth he introduced 

 the new generic name Margaritana. 



Thiele now claims that Oken was the first to split the old genus, 

 and that his names have the priority, while Haas claims that Bru- 

 guiere had the intention to split the old genus, and that he thus has 

 the priority over Oken. With reference to the latter claim, Thiele 

 says that there is no evidence whatever in Berguiere's paper to show 

 that he intended to divide the genus Unio. 



Both authors are right. Bruguiere may have had the intention of 

 dividing the genus, but there is no positive proof of it, and as long 

 as the dispute concerns the possible intentions of Bruguiere the ques- 

 tion will never be settled. But I should like to offer here two sug- 

 gestions which probably will help us. 



(1) Unio Retzius is a genus without a type, but with a diagnosis, 

 which includes species with and without lateral teeth. No matter 

 what the intentions of Bruguiere were, the fact remains that he, with 

 a full knowledge of Retzius' diagnosis, gave another diagnosis, in 

 which he mentions only the presence of lateral teeth. Thus, although 

 we cannot say that the genus Unio has been split or divided, surely 

 the concept has been changed and restricted. This is a perfectly 

 legitimate way in nomenclature, and thus Bruguiere introduced the 

 first change in the definition of the genus, and consequently has the 

 priority over Oken. Oken, according to our modern rules, had no 

 right any more to use Unio in such a way that it would exclude all 

 of Bruguiere's species. 



(2) Unio Retzius is a genus without a type, that is to say we do 

 not know which one of the six species enumerated by Retzius is the 

 type. Yet looking again at the diagnosis, we see it says that in 

 " most " species (in plurimis] lateral teeth are present. This makes 

 it perfectly clear that Retzius regarded the absence of lateral teeth 

 (in one species) as an exception to the rule, while the other five spe- 



