CH. Vl] 



THE FRESH -WATER MUSSEL 



165 



as possess the full complement of hinge-teeth those placed 

 beneath the umbones are termed " cardinal- teeth," in 

 front of these are the " anterior lateral-teeth," and behind 

 are the "posterior lateral." Unio, while possessing both 

 anterior and posterior laterals, has no cardinal-teeth 



p.m 



Fig. 29. The right valve of Anodonta cygnea from within. (After Howes.) 



If carefully dissected from the body under water, it will be seen that the 

 superficial chitinous layer is continuous on the dorsal side, between 

 the points i. n., and also for the area marked $', where it is turned 

 in and reflected on to the pallial muscle. The lines drawn in the 

 interior of the valve indicate impressions which mark the points of 

 origin, the shifting during growth and the final attachment of the 

 muscles. 



a. a. Anterior adductor muscle. a.p. Posterior adductor muscle. 



/</. Ligament. p.m. Pallial muscle. p.p. "Protractor pedis" 

 muscle (retractor of shell). r.m'. Anterior " retractor " muscle 



(protractor of shell). r.m". Posterior "retractor" rmiscle 



r.m'". Lesser "retractor" muscle (depressor 

 falve of the shell. s'. Inturned edge of shell. 

 u. Umbo. 



(protractor of shell). 

 of shell). s. Val 



