68 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 20 



usually is thinner and the hairs on the periostracum seem to be broader 

 and weaker than those found in M. modiolus. The posterior part of the 

 mantle seems to be more like M. capax than M. modiolus. A comparison 

 of the soft parts between 71/. modiolus and M. americanus, and between 

 Pacific and Atlantic specimens, is necessary before the question about the 

 allocation of west American specimens to 71/. americanus can be settled. 

 These specimens are, as stated above, so like some samples of 71/. ameri- 

 canus from Florida that it seems impossible to give a description which 

 can be used to separate the Pacific specimens. 



Occurrence: The only depth record noted is 5 fms. in Bahia de Santa 

 Maria. 



Distribution: Bahia de la Magdalena, Baja California (perhaps Laguna 

 de San Ignacio, Baja California, 71/. modiolus E. K. Jordan?) ; Mazat- 

 lan, Sinaloa, Mexico ; Isla La Plata, Ecuador. In the Atlantic from 

 North Carolina to the West Indies. 



Genus AMYGDALUM Megerle von Muhlfeld 1811 



Amygdalum Megerle von Muhlfeld, Gesell. Naturf. Freunde Berlin, 



vol. 5, 1811, p. 69. 

 Syn.: Modiella Monterosato 1884. 

 Type of genus: Amygdalum dendriticum Megerle von Muhlfeld 1811 



^ Mytilus arborescens Chemnitz 1795 (monotypic). 

 Remarks: The short generic diagnosis and the reference to Chemnitz, 

 PI. 198, figs. 2016 and 2017, clearly identify this genus. It is character- 

 ized by the thin modioliform shell, which is rather flat and furnished with 

 a shining periostracum. The posterodorsal part is usually more or less 

 spotted with white or gray, while the anteroventral side is uniform, some- 

 times blotched with yellow. Green colored species seem not to belong to 

 this genus. The resilial ridge is very thin and apparently compact ; the 

 nymphae are weak. 



The anterior adductor is elongate ; the posterior adductor is small and 

 round, placed high in the posterior part of the shell. The anterior retrac- 

 tor is slender and fastened in the umbonal cavity. The posterior retractors 

 have a thin anterior branch and a more solid posterior branch fastened to 

 the shell above the adductor. All retractors seem to go to the byssal gland 

 (cfr. Pelseneer, 1911, pp. 17-18, PI. 4, fig. 10; PI. 5, fig. 1). The mantle 

 margins are simple and a short septum separates the branchial part from 

 the anal opening. The animal builds a nest of mud and sand particles 

 held together by fine byssus threads. 



