4 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 20 



Volsella. Many species, however, have been placed in one of these genera 

 by some authors and in the other genus by other authors, indicating that 

 the outer form alone is of minor systematic value. The elongate boring 

 forms with parallel dorsal and ventral margins and subterminal umbones 

 were placed in the genus Lithophaga, though several of these species have 

 been considered to be Modiolus by some authors. 



The species with pronounced anterior and posterior radiating sculp- 

 ture separated by a smooth central part, the Alusculus-group, complete 

 the four major groups according to the shell outline and sculpture charac- 

 ters, which have been used for classification of the mytilids. When the 

 genus Brachidontes is added for the species with radiating sculpture over 

 the whole outer surface, and the three genera Idasola, Dacrydium, and 

 Crenella for the minute more aberrant species, the list is complete for 

 the genera used by Thiele (1935) for all recent species of the family 

 Mytilidae. Except for the last three mentioned, there have always been 

 difficulties in the allocation of many species to the proper genera. The 

 differences in opinion have been caused partly by the vaguely defined or 

 circumscribed genera and partly by the idea that all species should be 

 placed in the few "old" genera. During the years several supraspecific 

 categories have been named, but ordinarily they have been considered of 

 subgeneric rank and placed within the broad limits of the larger genera, 

 thus indicating close relationship based on external characters of ques- 

 tionable value. One thing seems certain, the system of the Mytilidae has 

 not been synthetized from small clearly circumscribed units, but was 

 built up by forcing species of apparently very different origin into large 

 groups of superficial similarity. 



The supraspecific groups should be built up and circumscribed in the 

 same way as is used for a species concept based on different populations. 

 As many characters as possible of the shell and animal should be studied, 

 also the distribution and if possible the history of the species considered, 

 when arranging them into supraspecific groups. If there are doubts as to 

 whether a species really belongs to a group or not, it is safest and most 

 correct to keep it apart until the right systematic position can be clearly 

 shown. If there are doubts about the relationship of several supraspecific 

 groups, it is safer not to press them into one large genus for the sake of 

 simplicity, but to keep them apart until this relationship can be proved 

 or disproved. The same idea should be applied to species. If specimens 

 from one geographical area are supposed only doubtfully to be identical 

 with a species from another area, it is safer to use a special specific name 

 for them until otherwise proved. The "new" species will naturally be 



