NO. 1 SOOT-RYEN : THE FAMILY MYTILIDAE 17 



Umbones usually terminal, radial sculpture on ventral part 

 unilaterally bifurcate, posterior retractors fastened below 

 the dorsal part of the posterior adductor .... Hormomya 



12. Mytiliform species with 1 to 3 tooth-like ridges or small 



teeth, anterior adductor obsolete or missing 13 



Anterior margin crenulated, anterior adductor distinct ... 14 



13. Margins smooth or with folds in young specimens, anterior 



retractor elongate, hinge with 1 to 3 tooth-like folds . Aulacomya 

 Margins crenulate, anterior retractor round, posterior re- 

 tractors broadly united with the posterior adductor, hinge 

 with several small teeth Ischadium 



14. Radial sculpture of equal strength over the whole surface, 



smaller greyish or yellowish-white species with thin perio- 



stracum 15 



Radial sculpture weaker or absent on the median part of the 



shell, periostracum strong 16 



15. Radial ribs relatively strong, a crenulated tooth-like process 



below umbo, ligament deep-set Crenella 



Radial striations numerous and fine, no tooth-like process 



below umbo in adult shells, ligament more marginal . . Solamen 



16. Radial sculpture distinct though weaker in the median part of 



the shell, anterior retractor in the umbonal cavity and a 

 small but distinct scar between the anterior adductor and 



retractor Arcuatula 



No radial sculpture on the median part of the shell, anterior 



retractor before umbo 17 



17. Umbonal keel low, periostracum without hairlike protuber- 



ances Musculus 



Umbonal keel pronounced, periostracum with long hairlike 



protuberances on the keel Gregariella 



18. Ligament short below umbo, small white or hyaline species 



with thin shiny periostracum 19 



Ligament elongate, larger species with strong periostracum . . 20 



19. Hinge without grooved teeth on the sides of the chondro- 



phore, anterior margin thickened Dacrydium 



Hinge with two grooved teeth with a small median chondro- 



phore, anterior margin not thickened Quendreda 



20. Shell elongate, lithophagiform with parallel dorsal and ventral 



margins, periostracum strong with irregular sculpture or 

 chalky or filthy incrustations, anterior retractor before 

 umbo 21 



