Occasional Papers of the Museum of Zoology 17 



Actinonaias sapotalensis (Lea) (1841), and approaching 

 subspecies computata (Crosse and Fischer) (1893). — Seven 

 specimens from Arroyo Hueyapam (H, vii, b) ; 8 from Rio 

 San Juan (H, vii, c). The former include the specimens of 

 which the anatomy has been described and figured by Ortmann 

 (1912). 



The specimens from the Arroyo Hueyapam are near sapo- 

 talensis, while those from the Rio San Juan approach compu- 

 tata, which, from the proportions and the figure of the hinge 

 armature, appears to be the description of a youngish indi- 

 vidual of a considerably larger form, from the Goatzalcoalcos 

 River system. One of the larger specimens is arcuate ventrad 

 very much like some of the specimens of P. opacata (plate 

 VII). In these older individuals, the wavy tendency of the 

 rays becomes accentuated towards the ventral margin, until 

 they are broken into very pronounced zig-zags, similar to some 

 of the species of Plagiola. 



Measurements 



Actinonaias umbrosa (Lea) (1856) and approaching expli- 

 cata ("Mo." C. and F.) (1894). 



U. alienigenus C. and F. (1893). Plate VII, figs. 43-47. 



Five adult shells and 15 juveniles from the Arroyo Hueya- 

 pam (H, vii, b) ; 12 specimens, including odd valves, from Rio 

 San Juan (H, vii, c). 



