18 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL.20 



Shell relatively higher, periostracum smooth or with hairlike 

 protuberances, anterior retractor usually behind umbo ... 22 



21. Shell coated with chalky incrustations especially on the post- 



erodorsal part ; margins smooth Lithophaga 



Shell with irregular sculpture or a filthy incrustation on the 



posterodorsal part, dorsal margin sometimes crenulated . Adula 



22. Dorsal margin crenulated above and behind the ligament, 



anterior adductor placed on the margin below umbo . . Botula 

 Dorsal margin smooth, anterior adductor not placed on the 



margin 23 



23. Shell mytiliform, lunule bordered by a faint line, anterior 



retractor elongate, behind umbo Semimytilus 



Shell with broader rounded anterior margin, anterior re- 

 tractor rounded 24 



24. Anterior retractor in or behind umbo 25 



Anterior retractor before umbo 26 



25. Greyish or yellowish-white, usually spotted, species with shiny 



periostracum; first posterior retractor small, widely sepa- 

 rated from the hinder one Amygdalum 



Brownish species with thick periostracum often with hairs, 



posterior retractors continuous Modiolus 



26. No furrow separating the posterodorsal area from the rest of 



the shell Lioberus 



A furrow from umbo to the posterior part of the ventral 



margin is present Musculus 



Genus MYTILUS Linne 1758 



Mytilus Linne, Systema Naturae, ed. 10, 1758, p. 704. 



Syn.: Eumytilus Ihering 1900. 



Type of genus: Mytilus edulis Linne 1758 (subsequent designation by 



Gray 1847). 

 Remarks: This Linnean genus, which originally contained several very 

 unlike shells now referred to different families, is here restricted to those 

 species which have the features characteristic of Mytilus edulis Linne. 

 These are mainly the anterior position of the umbones, the dysodont 

 teeth on the short anterior margin formed by the radial ridges of the 

 lunule, a small anterior adductor, elongate scars of the anterior retractor 

 well behind the umbones, continuous scars of the large posterior adductor 

 and retractors, and the distinctly pitted resilial ridge which fastens the 

 ligament to the nymphae. The posterior part of the mantle margins are 

 furnished with tentacles or papillae. 



