126 THER&TES. 



tuberculate ; tail with a slight median longitudinal groove above 

 (pi. 33 figs. 6, 7. T. gulosa Old.). 



Jaw arcuate, stout, with 5 to 12 unequal, strong ribs (pi. 32, 

 figs. 47, 48, 50). Teeth having the side cusps of centrals and inner 

 laterals completely fused with the middle cusps ; marginals having 

 a long bifid inner cusp (entocone plus mesocoue) and a simple or 

 bifid ectocone (pi. 34, fig. 1, T. mitcheUce). 



Genital system having no accessory organs on the female side, 

 the duct of the spermatheca generally long and swollen below. 

 Penis enlarged distally, where its cavity contains a solid papilla ; 

 epiphallus bearing the retractor, and terminating at the entrance of 

 the vas deferens in a short flagellum (pi. 33, fig. 1, Thersites. 

 richmondiana, and figs. 2, 3, T. mitchellce. PL 51, fig. 10, T. soloren- 

 *i8). In some species the epiphallus is shortened and the flagellum 

 very short or absent by degeneration (pi. 32, fig. 52, T. pachystyla, 

 and fig. 51, T. rainbirdi). 



Habits strictly terrestrial. With the exception of a few New Gui- 

 nea species, and some inhabiting the Timor group, the species of 

 this genus are confined to Australia, where they are generally dif- 

 fused, everywhere constituting the most prominent feature in the 

 Helix fauna. 



The various sections assembled under the generic term Thersites 

 form a very homogeneous group, the extreme forms being well con- 

 nected by a chain of intermediate species, Xantliomelon and Rhacjudu 

 forming outlying or satellite groups of slightly greater systematic 

 value than the other sections, but still intimately allied. The shell 

 varies from thin, light forms like corneovirens through a series of 

 transition species to the solid, richly dyed blomfieldi, mitchelhe and 

 bipartite; and by other chains of almost unbroken continuity, the 

 globose forms are connected with the keeled rickmondiaua and 

 kooringensis. The soft anatomy fully sustains these conclusions. 



The genus Thersites is allied to Chlortiis, and might without any 

 great violence be united to that genus ; but it will probably prove 

 an aid to clear and correct thinking to retain the two separate. 

 Thersites never has the depressed earlier whorls, or quincuncially 

 arranged hairs or spaced points so characteristic of Chlor'dis, and 

 the flagellum is shorter or obsolete. 



The distribution of the Thersites and Chtoritis groups seems to 

 indicate a hypothesis of two separate times of connection between 

 Australia and the Papuan tract since the beginning of the Tertiary. 



