NO. 1 SOOT-RYEN : THE FAMILY MYTILIDAE 15 



The mytilid form seems to be successful for species living in colonies 

 fastened with a byssus to rocks or seaweeds. In the pointed anterior end 

 is only a small space for an anterior adductor which, therefore, has a ten- 

 dency to become reduced. To obtain a rapid and complete closing of 

 the valves, the posterior adductor is larger and the posterior retractors 

 of the byssus are strong, so the shell can be pressed against the support. 

 Strong retractor muscles need space for the adherence to the shell and 

 are therefore found in a long flattened band before the adductor or 

 separated into two strings. Usually the narrow anterior margin is fur- 

 nished with tooth-like folds formed by the radiating sculpture of the 

 lunule. The sculpture seems to be the remains of a radiating sculp- 

 ture in the ancestors, which perhaps were like modern species of the 

 Brachidontes or the Hormomya-group. In small species, e. g., of Mytilus 

 s. S.J the teeth are not interlocking but are separated by the periostracum 

 bent inwards in both valves. In other groups where the lunule is more 

 or less completely bent inwards', the teeth, or more correctly the folds, 

 interlock. 



The posterior part of the mantle margin, the branchial opening, is 

 furnished with tentacles or papillae of various forms usable as filtering 

 and sensory organs. Nearly all mytiliform species seem to have such 

 papillae. The dorsal or anal opening is sometimes in the form of a short 

 siphon, sometimes an opening in the septum connecting the two mantle 

 margins. 



As stated before, all mytiliform species have been included in the 

 genus Mytilus s. L, though representing different subgenera or sections. 

 The differences which can be seen inside the valves or in the soft parts, 

 are in reality greater than those used for the separation of genera or even 

 subfamilies in other pelecypod families. In this paper two new generic 

 names are introduced for species differing considerably from Mytilus s. s., 

 one of which is not represented in the recent fauna of west America. 

 The grouping together of the five genera of mainly smooth mytiliform 

 species seems at present to be unwise. Many students and especially 

 the palaeontologists certainly will prefer to use Mytilus for all these 

 species on the basis of the outer form, but in that case Aulacomya has 

 also to be included. That such a procedure will clarify and simplify the 

 relationship of these species seems questionable. The heading of this 

 section does not indicate a relationship of mytiliform species a priori 

 or that these genera are more closely related to each other than to other 

 genera of the family, but only that they have been supposed to constitute 

 a uniform group. 



