HANNIBAL: CALIFORNIAN FRESHWATER MOLLCSCA. 121 



recent representatives, indicate a primitive phase of Naiad structure 

 in so far as the anatomy is concerned. 



Margaritana, the only recognized genus, appears to be confined to 

 the Northern Hemisphere. However, M. monodonta, an American 

 species, should doubtless be separated as a distinct genus ; it differs 

 in important particulars from M. margaritifera in the adolescent shell, 

 a certain indication of heterogeneous origin. 



Genus Margaritana, Schumacher. 



i/y« (sp.), Linne, 1758 (if. margaritifera^ L.) ; Unio (sp.), lletzius, 

 1788 {2[. margaritifera, L.); Margaritana, Schumacher, 1817 

 (J/, margaritifera, L.) ; Alasmodonta (sp.), Barnes, 1823 {A. 

 arcuata, Barnes = i!/". margaritifera, L.); Damaris, 'Leach MS.,' 

 Gray, 184 7 (J/", margaritifera, L.), in synonymy; Bapliia, 

 ' Meuschen,' H. & A. Adams, 1857 {M. margaritijera, L.). 

 Type, Mya margaritifera, Linne. 



Sub-genus Pseudunio, Haas. 



Lynmium (sp.), Moquin-Tandon, 1855 [JJ. sinuafa, Lam.); Pseudunio, 

 Haas, 1910 ( Z7. sinuata, Lara.). 



Type, TJnio sinuata, Lamarck. 



Sub-genus Pseudunio. Shell of moderate size, averaging 120 mm. 

 in length, sub-solid, elongate-elliptical, umbones marked by a few fine 

 sub-nodulous ridges, moderately elevated and lying about one-third of 

 distance from anterior extremity, lateral teeth fairly developed ; nacre 

 of concavity mai'ked with numerous small muscle-scars ; habitat 

 lacustro-fluviatile and fluviatile. 



Sub-genus Margaritana, s.s. Shell similar to preceding, but the 

 lateral teeth largely or entirely obsolete ; Pseudunio stage completed 

 very early during adolescence ; habitat running streams. 



Margaritana (Pskudunio) Herrei, n.sp. PI. VII, Fig. 17. 

 Margaritana '■margaritifera, Linn.', Walker, 1910 [-partim). 



Shell rather large for genus, resembling M. margaritifera in general 

 outline, but narrower and more compressed, with a straighter dorsal 

 line ; teeth not obsolete, two clavate pseudo-cardinal and two laterals 

 of moderate length in right valve, one each in left ; habitat apparently 

 lacustrine. Length 115, breadth 40, depth of valve 12 mm. 



Eocene : local freshwater beds in Tejon formation, California. 



One -fourth mile above Carnegie Pottery plant, in cut along 

 Western Pacific Ilailway, Corral Hollow, Tesla, California (Stanford 

 University Geological Survey, per W. H. Ochsner) (H. Hannibal). 



Named after Dr. A. C. Herre, under whose guidance the writer 

 first became interested in the study of molluscs. 



Margaritana margaritifera (Linne). 



Mya margaritifera, Linne, 1758; Alasmodonta arcuata, Barnes, 1823 

 (syntonic form). 



