LOWER CALIFORNIAN MOLLUSCA. 343 



Helicodiscus lineatus sonorensis n. subsp. Plate 

 xiv, fig. 10. 



The little shell here figured was found by Dr. Eisen 

 in a damp locality near San Miguel, Sonora, Mexico, and 

 supposed at first to be the young of a Planorboid aquatic 

 shell, but the microscope proved it to be probably a young 

 shell of Say's species, or perhaps a regional subspecies 

 to which I have given tne above provisional name. The 

 figure loa would prove its immaturity if it did not have a 

 slightly thickened lip, there being but three whorls visible 

 and no teeth. H. lineatus is one of the most widely 

 spread species of North America, from New Brunswick, 

 latitude 49°, to Montana, and Georgia to California, but 

 with varieties or subspecies having different characters. 

 That of Sonora may, when found mature, add another 

 form, but this specimen seems only about half grown. 



It differs from lineatus in the subangled margin, 

 smaller umbilicus, more rapidly enlarging whorls, and 

 no denticles. The Californian subspecies is also without 

 denticles. It is barely possible that a new Planorbis 

 may occur in Mexico having these characters, but none 

 is known elsewhere with bands beneath the epidermis. 

 I have compared it with Gould's P. kirsutus {P. albus 

 Muller var.f). This resembles the Sonora shell in hav- 

 ing similar lines in the epidermis, but they produce rows 

 of bristles which fall off easily with the skin, leaving no 

 marks. It is also one-fifth larger than H. lineatus and 

 has only 3 whorls when full grown. 



It is somewhat singular that Say, the author of the 

 species, made the mistake of describing a specimen as a 

 Planorbis. It was found " in a dried up pond with a 

 number of aquatic shells in the Upper Missouri region." 

 He called it P. -parallelus, and, although three times as 

 large as the Sonora specimen, he did not suspect its true 



