8 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 20 



wards and slightly into the mantle cavity. The anal siphon has smooth 

 edges in the species which have been studied. Below the contiguous part 

 and the septum there is a part, sometimes elongate like a siphon but open 

 ventrally, where the mantle edges may be furnished with tentacles or 

 papillae of varying shape, or may be smooth. This is the incurrent or 

 branchial part of the mantle opening (cfr. text-fig. 10). 



In the present paper some anatomical characters which are easily seen 

 or which can easily be studied even in empty valves have been mentioned 

 for most of the species. The posterior part of the mantle margin is always 

 described and drawn from preserved animals and may vary slightly in 

 different specimens according to the degree of contraction. The major 

 features are, however, believed to be correctly shown in the drawings. 

 A thorough anatomical survey of the whole animal will certainly be 

 needed before definite conclusions as to the relations of the various 

 supraspecific groups can be made. 



Despite this weakness and certainly some wrong conclusions, it is hoped 

 that this study of a few mytilid species will clarify some questions and 

 be of help for future study. 



NOMENCLATURE 



The name of a species or a supraspecific group has only one purpose, 

 namely, to be referable to one well circumscribed unit of animals. A 

 scientist should know the supraspecific groups of his specialty so well 

 that on hearing the name of a species unknown to him, he can imme- 

 diately visualize it by comparison with well known species of the same 

 group. If a supraspecific group is made very wide, however, and there- 

 fore vaguely circumscribed, the scientist has to know each species to 

 remember the characters peculiar to it. 



By a strict application of the International Rules of Nomenclature, 

 many names, familiar for decades to scientists from some parts of the 

 world, have to be changed. This procedure is inconvenient for present 

 day scientists, but it will not be more difficult for future students than 

 the use today of papers one hundred years old. There have always been 

 different opinions about the scientific names and so numerous changes 

 have been made during the years. Certainly these changes are of no 

 benefit to systematics and taxonomy, and if they could be stopped by 

 fixing names to good descriptions and illustrations, with international 

 cooperation, it would be a great benefit to systematists and to zoologists 

 in all branches. 



