PANAMIC-PACIFIC PELECYPODA HI 



the other and sometimes wholly eliminated in the adult, hence the posterior 

 adductor scar is large and sometimes confluent with that of the posterior 

 retractor. Surface smooth or with radial riblets; the sculpture is usually- 

 much stronger on the posterior-dorsal side, finer on the ventral side of the 

 umbonal slope. Inner layer of the shell is ordinarily nacreous, the outer layer 

 thinner, darker, and covered by a thin or heavy, yellow, brown, green or 

 black periostracum, hairy or bristly, often peeling off from dead valves. 



Marine, brackish to fresh water. Most species live in colonies, the shell 

 anchored by its byssus to rocks, piling or to sea-weeds. Some species form 

 nests of byssus in which they lie buried. Some species are rock borers such 

 as the Lithophagas and Adula. 



Earlier authors distributed the species of this family amongst two key 

 genera, Mytilus and Modiolus, principally from the position of the beaks, 

 whether entirely terminal, the anterior end of the shell therefore more or 

 less pointed, or with the beaks more remote, the anterior margin projecting 

 a space in front of them. All transitional forms occur between these two 

 types and many species have, at different times and by the same author, 

 been referred to both genera. 



The reader is referred to the recent important paper of Tron Soot-Ryen, 

 1955, A Report on the Family Mytilidae: Allan Hancock Pacific Expedi- 

 tions, vol. 20, No. 1 for much additional information on this family. 



Key to Panamic-Pacific genera 



I. Resilifer lined with a chalky white layer, its basal part cellular, hence 

 showing as a line of pores or pits on its exposed edge. 



1. Shell mytiliform, the beaks strictly terminal, pointed. Anterior or 

 lunular side below the beaks with a few, short, riblike folds terminating 

 in strong teeth within the margin. Mostly cold water and northern. 



Genus Mytilus 



2. Shell modioliform, the beaks a little removed from the end, the anterior 

 margin projecting beyond them. No folds or teeth on the anterior side 

 and margin. 



Genus Mytella 



II. Resilial layer white, compact and showing no pores or pits on the side. 



A. Surface smooth or marked with concentric growth lines (except 

 Aulacomya, which has variable ribs on umbones), the margin of the 

 valves plain or smooth. 



a. Individuals free or nestlers, not boring into rock, coral or shell. 



aa. Shell mytiliform, the beaks terminal and pointed. 



3. Often large, externally like Mytilus, the anterior adductor scar obsolete 

 and the hinge teeth reduced to a single, riblike tooth. 



Genus Choromytilus 



4. Similar but with a persistant anterior adductor scar and generally with 

 a series of low, radial ribs or streaks on the umbones. Peruvian. 



Genus AulacoTnya 



