522 PATAGONIAN EXPEDITIONS I ZOOLOGY. 



Length n, diam. 6.1, length of aperture 6.3 mm.; whorls 



9-8 " 5-5 " " 5 



8.8 " 6 " " 5.3 " 



Family 



LYMN^A Lamarck. 



Lymncea is more widely distributed than any other genus of freshwater 

 mollusks, extending from the shores of the Arctic Ocean to Tasmania and 

 Cape Horn, and in the Pacific reaching the Hawaiian group. The genus 

 in its present limits is a synthetic group, which no doubt will ultimately 

 be divided into several genera. 



There are very few species in tropical South America, where the genus 

 seems to be of rare occurrence ; but in Patagonia the species are more 

 numerous, individuals are abundant and generally distributed. South 

 American Lymnaeas fall into three subgenera or sections of the genus. 

 Section I is clearly an intrusive element from North America. Sec- 

 tion II may possibly be of North American origin, but its relationships 

 are unknown, as no specimens with the soft parts have been received. 

 Section III is peculiar to Patagonia, and not closely related to any nor- 

 thern forms. 



I. Section GALBA Schrank. The marginal teeth of the radula differ 

 from the laterals by being more oblique, but are essentially tr^ispid. The 

 shell is small, compactly coiled, of very convex whorls, usually umbilicate, 

 the columellar lip broadly revolute, not folded. The type is L. truncafala 

 of Europe. South America species, L. viator, L. cousini. 



II. Section ? Dentition unknown. The shell is lengthened, 



fragile, Succmea-shaped, of few whorls, the last large and elliptical. L. 

 peregrina, L. andeana. 



III. Section PECTINIDENS n. sect. The marginal teeth are broad and 

 short, the cusps split into a comb-like series of many narrow denticles. 

 Type Lymncea diaphana King. There are two groups of species. 



Group of L. diaphana. Moderately large, elongated forms, with very 

 convex whorls and a narrow umbilical chink. 



Group of L. patagonica. Small forms, with very short spire (peculiarly 

 liable to truncation) and few whorls, the last relatively very large and 

 short, rimate or imperforate. L. patagonica, L. p. riochicoensis, L. pic- 



