YON JEERING : ON SOUTH AMERICAN MYTILID .E. 87 



2. Subgen. Chloromya, Morch (1853). 



Surface smooth, or concentrically sculptured. Anterior adductor 

 absent, byssal muscle separated from the posterior retractor muscle. 

 Colour often yellowish, or green, with darker zigzag markings. 

 Umbones terminal. Type, II. pema, Linn. 



3. Subgen. Trichomya, n.subg. 



Sbell radially striate, or smooth with oblique decussate lines, and 

 fibrous-hirsute periostracum. Umbones terminal. Teeth developed, 

 or obsolete. Type, II. hirsutus, Lam. 



I propose this section for II. hirsutus, II. horridus, Dkr., and 

 M. tortus, Dkr. Tbe last-mentioned is the type of Gray's subgenus 

 Stavelia, which I do not accept, because it separates II. tortus, 

 which is twisted, from II. horridus, its nearest ally, although not 

 twisted. 



4. Subgen. Mytilaster, Monterosato (1884). 



Shell smooth, concentrically sculptured, and sometimes exhibiting 

 faint indications of radial sculpture. Dorsal margin denticulate. 

 Umbones subterminal. Type, II. minimus, Poli. 



M. Solisianus, Orb., which belongs to this section, sometimes shows 

 indications of a very faint radial sculpture, differing very little from 

 II. minimus. 



5. Subgen. Aulacomya, Morch (1853). 



Shell strongly radially ribbed, umbones terminal. Anterior adductor 

 often absent. Type, II. Ilagellanicus, Chem. 



To this group belong II. exustus, Linn., If. atropurpureus, Dkr., 

 II. decussatus, Lam., II. hamatus, Say, etc. 



6. Subgen. BIormomya, Morch (1853). 



Shell radially sculptured, umbones subterminal. Anterior adductor 

 present. Type, II. Domingensis, Lam. 



In this section, with the radiate Mytili I place also the radiate 

 Modiolae with subterminal umbones forming the subgen. Brachydontes, 

 Sws., of which II. citrinus, Bolten (= sulcatus, Lam.), is the type. 



Genus MODIOLUS, Lam. (1799). 



(= Modiola, Lam., 1801.) 



Shell equivalve, inequilateral, umbones non-terminal, marginal. 

 Anterior adductor present. Cardinal teeth absent or rudimentary. 



1. Subgen. Eumodiolus, n.n. (= Modiolus, s.s.). 



Shell inflated, surface smooth, periostracum not polished, often 

 hirsute. Type, II. modiolus, Linn. 



