190 OXYCHONA. 



The prominent features of this group are the smooth, thin, light- 

 colored and banded shell which is usually of a markedly trochiform 

 contour, but sometimes depressed, the periphery angular ; the ribbed 

 jaw ; the extremely peculiar radula, with enormously widened and 

 enlarged middle cusps, and minute, basal side cusps. When the 

 radula is torn, the teeth part readily along their lateral faces, but 

 adhere in chain-like longitudinal rows. 



The radula, as well as the jaw and shell, is comparable to that of 

 Papuina (cf. p. 137, pi. 37, fig. 10), but although the superficial re- 

 semblance is great, the two are really totally distinct, the broad 

 cusps of Papuina being formed by the united ento-, meso- and ecto- 

 cones, whilst in Oxychona the mesocone only is modified, the side 

 cusps becoming obsolete. Polymita also has a slightly similar but 

 morphologically different dentition. The peculiar type of teeth in 

 these three genera has evidently been independently evolved in 

 each, from the usual tricuspid type. It seems to be correlated with 

 arboreal habits. Compare also the radula of Otostomus. 



The affinities of Oxychona are uncertain ; but it will probably 

 prove to be a member of the Belogona, distinguished from Helix 

 by its Papuina or Corasia like shell, and the peculiar teeth. Prob- 

 ably in this group, as in Papuina, some species will be found to 

 have a more normal type of dentition. The Mexican forms which 

 have been placed in this group are still unknown anatomically, but 

 the animal of 0. trigonostoma has been figured with a toothed keel 

 on the tail, such as occurs in the genus Lysinoe. 



Messrs. Crosse & Fischer proposed the section Leptarionta for two 

 species, bicincta and flavescens ; but as they state that they had not 

 seen flavescens, their group was evidently founded on bicincta, the 

 first species described by them. 



The first four species are from Brazil ; the others are from 

 Guatemala and Costa Rica to the province of Vera Cruz, E. 

 Mexico. 



O. bifasciata Burrow, v, 129. O. zhorquinensis Ang., v, 132. 



pyramidella Spix, Wagner. O. trigonostoma Pfr., v, 132. 



bosciana FIT. /. elevatoconica C. & F. 



blanchetiana Moric. lalliana Tristr. 



O. lonchostoma Mke., v, 130. /. salleana Pfr. 



O. gyrina Val.,v, 131. obscura C. & F. 



O. pileiformis Moric., v, 131. /. intermedia C. & F. 



