190 The MoUuscan Family Planorbidae 



Species Considered as Valid. There are several distinct species and 

 races of the subgenus Micromenetus recognizable within the genus Menetus, 

 as follows: 



Menetus (Micromenetus;) dilatatus Menetus (Micromenetus) alabamerisis 



dilatatus (Gould) aims Pilsbry 



Menetus (Micromenetus) dilatatus penn- Menetus (Micromenetus) brogniartianus 



sylvanicus Pilsbry (Lea) 



Menetus (Micromenetus) dilatatus Menetus (Micromenetus) sampsoni 



buchanensis (Lea) (Ancey) 



Menetus (Micromenetus) alabamensis Menetus (Micromenetus) uliginosus 



alabamensis Pilsbry Vanatta 



Geological Distribution. Not exactly known but certainly from the 

 Pliocene to Recent fauna. 



Remarks. The group here separated as Micromenetus differs from 

 typical Menetus in the size of the shell which is always much smaller, none 

 exceeding 4 mm. in diameter. The form of the shell is lenticular and there 

 is usually a peripheral carina more or less well develoiied. The penial gland 

 has a duct which is almost three times as long as the gland and is attached 

 to the inner wall of the preputium for the greater part of its length (plate 

 40, fig. 10). In typical Menetus, this duct is short and enters the diaphragm 

 directly without being attached to the wall of the preputium (plate 39, fig. 

 7). Tlie pseudobranch in Micromenetus is also very long and narrow 

 while in typical Menetus it is short and wide (compare jilate 38, fig. 14, 

 with plate 40, fig. 9). These are small differences, perhaps, but they appear 

 constant. Micromenetus differs from both Promenetus and Planorbula in 

 the shape of the penial gland. As far as examined the radulae of the two 

 groups differ in formulae, that of Menetus being 20-1-20 while in Micro- 

 menetus it is 15-1-15. 



Genus PLANORBIFEX Pilsbry, 1934 

 Type by original designation Planorbis vanvlecki Arnold 



1934. Planorbijex Pilsbry, Proc. Acad. Nat. Sci. Phil., 86, p. 568. Genotype Planorbis 



vanvlecki Arnold. As subgenus of Menetus 



1935. Menetus (Planorbijex) vanvlecki Pilsbry, J. Henderson, Fossil Non-Marine 



Moll. N. A., p. 252. 



Shell (plate 81, fig. 6). Discoidal, dextral in appearance, the right side 

 moderately convex, with the last whorl carinate, the periphery rounded; 

 left side deeply umbilicate, the last whorl more or less flattened. Sculpture 

 of fine, close, hair-like striae. Aperture strongly oblique, its outline excised 

 bv the preceding whorl, the receding left margin thickened within 

 (Pilsbry) . 



Geological Horizon and Distribution. Basal part of the Tulare forma- 

 tion. Pliocene. North dome, Kettleman Hills, Kings County, California. 



Species Considered as Valid. The fossil species Planorbijex vanvlecki 

 Arnold, as genotype, is the only species known for this genus. 



Remarks. 'Except for the narrow umbilicus, the absence of a peripheral 

 keel, and the thickened peristome, this form resembles Platytaphius 

 Pilsbry, a much larger planorb from Lake Titicaca, which has similar 

 sculpture. Paraplanorbis Hanna resembles Planorbijex in being small and 



