86 PEOCEEDINGS OF THE MALACOLOGICAL SOCIETY. 



I noted on M. perna, M. meridionalis, and M. hamatus, Say. The 

 adductor, in its primitive form, is only a modification of the fibres of 

 the united mantle lobes, and its isolation represents a later phylogenetic 

 state, which, perhaps, may be repeated in the ontogeny. Probably 

 the dimyarian state represents the older one, and the reduction and 

 disappearance of the anterior adductor a secondary modification, which 

 may have originated independently in many families. In the genus 

 Mytilus, the want of the anterior adductor is not a very important 

 character. Mytilus MageUanicus, Chem., from the Magellan Straits, is 

 dimyarian when young, but monomyarian in old specimens, while the 

 New Zealand race seems to be always dimyarian. 



Judging from my experience among the radially striate Mytili, 

 monomyarian forms occur only in the section Aulacomya, in which 

 Morch includes M. MageUanicus, Chem., and M. crenatus, Lam., from 

 South America (this last may be M. decussatus, Lam.). To this group 

 M. hamatus, Say, with acuminate anterior extremity and terminal 

 umbones, also belongs. On the other hand, the radially striate Mytili 

 of Morch' s section Hormomya, as also the striate Modioli, subg. 

 Brachydontes, are dimyarians. Since almost all these forms have the 

 anterior extremity obtuse, and the umbones subterminal, situated above 

 or very little behind the anterior extremity, it would seem natural 

 to accept for these species Morch' s subgenus Hormomya ; but, in 

 opposition to such a division, we must note that M. exustus, Linn., 

 has the pointed anterior extremity of M. hamatus, and it seems an 

 unnatural grouping to separate them because one is monomyarian 

 and the other dimyarian. 



It appears to me, therefore, convenient to separate the two groups 

 of striate Mytilus, one with pointed anterior extremities and terminal 

 umbones, the other with blunt anterior ends and subterminal umbones, 

 accepting in a modified sense the subgenera of Morch. 



I would propose the following division for the genera of Mytilus 

 and Modiolus, premising that since the collections in our Museum do 

 not allow me to make the work complete, the scheme can, therefore, 

 only be taken as affording an exact definition of the conchological 

 characters of the majority of the subgenera. 



Genus MYTILUS, Linn. 



Shell equivalve, inequilateral, umbones terminal or subterminal. 

 Cardinal teeth few, small, sometimes obsolete. Anterior adductor 

 muscle sometimes wanting. 



1. Subgen. Eumytiltjs, 1 n.n. (= Mytilus, s.s.). 



Surface smooth, or concentrically sculptured, umbones terminal. 

 Type, M. edulis, Linn. 



I propose to always distinguish those sections which have the same name as the 

 genus by prefixing " eu." 



