58 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 20 



Jousseaume has, according to Lamy (1919) established three new- 

 genera for various types of Modiolus. Genus Modiolatus Jousseaume 

 (1893) was erected for Mytilus plicatus Chemnitz and for several other 

 species; but as M. plicatus Chemnitz 1785 (p. 153, PI. 82, figs. 733 a 

 and b) does not seem to be a mytilid, this genus is very questionable. 

 Genus Arcuatula (Jousseaume) Lamy 1919, made for species with radi- 

 ating sculpture (Mytilus demissus (Dillwyn) 1817), is considered to 

 constitute a valid genus and is here treated as such. Genus Fulgida 

 (Jousseaume) Lamy 1919, was established for Perna fulgida H. Adams 

 (supposed to be synonymous with Modiola lignea Reeve by Lamy, with 

 Modiola philippinarum Hanley by Iredale, 1939). Jousseaume wrote 

 (cited by Lamy) that Fulgida was characterized by the glossy posterior 

 and dorsal parts, while the ventral region, which usually is glossy and 

 shining in Modiolus, is dull. This group may perhaps be used when the 

 Fulgida of Jousseaume has been critically studied. 



The west American species of Modiolus are partly of the typical form 

 and partly with a straight hinge line having a more or less distinct dorsal 

 angle. All are at present included in the genus Modiolus s. s. Volsella sal- 

 vadorica Hertlein and Strong 1946 is not a Modiolus and is provisionally 

 referred to the genus Lioberus Dall. One species is considered to be 

 unnamed and is here described as Modiolus neglectus n. sp. 



Young specimens are sometimes very difficult to refer to a particular 

 species. The young specimens of neglectus and eiseni are rather easy to 

 determine, while small sacculifer, fornicatus, modiolus, and sometimes 

 capax, may be very difficult to separate. Small specimens with long 

 smooth periostracum hairs may perhaps represent still another unrecog- 

 nized species, but the material at hand is unfortunately too limited to 

 allow a settlement of this problem. Carpenter (1855-57) says that very 

 young specimens of capax have long smooth hairs which become relatively 

 shorter and serrate. If this really is the case, young capax also can easily 

 be confused with young specimens of other species. As there seems to 

 be no safe way to name small specimens at present, they are not con- 

 sidered in this paper. 



One specimen from Ecuador shows strong affinities to the Atlantic 

 Modiolus americanus (Leach) and is listed tentatively under that name, 

 together with specimens from Mazatlan and Bahia de la Magdalena 

 found in the collections of the California Academy of Sciences. 

 Key to the west American species : 

 1. Periostracum with serrate hairs, left 

 valve usually more inflated than the 



