NO. 1 SOOT-RYEN : THE FAMILY MYTILIDAE 



INTRODUCTION 



It is desirable to start with a diagnosis of the family Mytilidae, though 

 only a few species from a limited area are treated in this paper. 



"Shell equivalve, generally very inequilateral with prosogyre umbones 

 near the anterior end ; ligament elongate, deep seated, generally on nym- 

 phae, the inner resilial part typically connected with the nymphae by a 

 calcified white ridge. Shell surface showing three areas : ( 1 ) the anterior 

 area, the lunule, more or less distinctly circumscribed, often with radiat- 

 ing sculpture and dysodont teeth on the inner margin; (2) the median 

 part, generally glossy and rarely with radiating sculpture; and (3) the 

 posterior part often strongly sculptured or set off by a different color or 

 by a keel. Shell ordinarily with the interior layer margaritaceous. Perio- 

 stracum strong, sometimes with hairlike protuberances or with incrusta- 

 tions. Prodissoconch with provinculum, which persists in some smaller 

 species. ]\Iantle lobes united below the anal siphonal opening, branchial 

 opening confluent with the pedal opening, posterior part of mantle edges 

 often pigmented and furnished with papillae. Pallial line simple or with 

 a shallow posterior concavity. Anterior adductor muscle smaller than 

 the posterior one, sometimes obsolete in adult shells. Anterior byssus- 

 retractor small, fastened before or behind the umbones. Posterior retractor 

 is generally confluent with the posterior adductor. Foot finger-shaped 

 with a ventral furrow, byssal gland behind the foot functional in most 

 of the species. Gills filibranch ; ventricle embracing the rectum, monoe- 

 cious. Marine ; a few species in brackish or fresh water." 



Generally, species of the family Mytilidae are easily recognized as 

 members of the family by the form, sculpture, hinge, and muscle scars. 

 Some small smooth species have a transverse striation on the dorsal hinge 

 margin like that found in the family Philobryidae, but the shell is not 

 pitted by the fine tubules typical of the shells of the latter family. 



It is difllicult to indicate satisfactory characters for the separation of 

 the family Modiolopsidae (fossil Ordovicium to Devonian) and the 

 family Mytilidae, which reaches back in time to Devonian. The most 

 logical explanation is that the Mytilidae form a continuation of some 

 branch of the Modiolopsidae. The Mytilidae have generally been divided 

 in groups according to the outer shape of the valves. Species with terminal 

 or nearly terminal umbones, the mytiliform species, have been placed in 

 the genus Mytilus, while species with subterminal umbones with a distinct 

 anterior margin, the modioliform species, have been named Modiolus or 



•WV 1 5 1955 



