NO. 1 SOOT-RYEN : THE FAMILY MYTILIDAE 7 



of special importance is the retractor muscles of the foot and byssus. 

 These muscles, which carry the visceral mass and can contract it against 

 the shell and move the foot, should certainly be the most important ones 

 for bivalves as well as for univalves. Whatever the origin of the pelecy- 

 pods may have been, as a larval partial duplication from a gastropod 

 origin or directly from the hypothetical ancestors of the mollusks, 

 retractor muscles must have been the primary muscles of the animal. The 

 adductors, usually the strongest muscles of an ordinary bivalve, seem to 

 be enlarged parts of the muscles fastening the mantle to the shell. The 

 position of the retractor of the foot compared to the posterior retractor 

 of the byssus, and the form and placement of the anterior retractor in 

 regard to the umbo, give valuable clues for classification (cfr. text-fig. 

 15). 



The shell surface of most of the mytilid species is divided more or 

 less distinctly into three areas, either by different sculpture or coloring. 

 The anterior part, the lunule, usually is radiately ribbed, grooved, or 

 striated, the grooves often forming teeth on the anterior margin ; or the 

 lunule is turned inwards so that the ridges form tooth-like thickened parts 

 with corresponding grooves in the opposite valve. Sometimes a lunule is 

 seen as a duller part of the periostracum (cfr. text-fig. 47), but often 

 it is not separable from the rest of the shell. The median part generally 

 is smooth and furnished with a more shining periostracum than the rest 

 of the shell. The posterior part, usually bordered by a more or less dis- 

 tinct keel from the umbones to the posteroventral angle, is more heavily 

 sculptured, has a different color, or hairy periostracum, or differs other- 

 wise from the rest of the shell. The margins may be smooth or crenulated 

 more or less extensively, generally in accordance with the sculpture. 

 Several species have margins partly crenulated when young but smooth 

 when full grown. The ligament is deep-set and a real escutcheon is not 

 developed. The ligament is supported by a sometimes solid nymphae and 

 the lower or ventral fibrous part, the resilial part, furnished with cal- 

 careous needles, is fastened to a ridge, here named the resilial ridge, 

 which is white and of a quite different consistency than the rest of the 

 valves (cfr. PI. 5, fig. 23). This ridge continues posteriorly to the dorsal 

 margin and forms one of the best characteristics of the family Mytilidae. 



The mantle is usually separated for the entire length of the shell, 

 from the anterior adductor to the middle of the posterior margin. An 

 excurrent opening, a slit in the septum connecting the two mantle lobes 

 or a more or less distinct tubuliform siphon, is separated from the free 

 mantle lobe by a short contiguous part and a septum extending ventral- 



