6l2 PATAGONIAN EXPEDITIONS: ZOOLOGY. 



— reach far back in Palaeozoic time. We have no data bearing upon either 

 the time or place of these events. 



I. We have first to do with a fauna composed mainly of the Orthure- 

 throus land-snails^ — forms which are structurally but a step removed from 

 the aquatic pulmonates, and now forming an insignificant element in conti- 

 nental faunas, though still dominant in the islands of the central Pacific. 

 The families ValloJiiidcB, Enidce, Pupillidce, Pavtulida, Fentssacidcc, 

 AniastridcB, AchatiiielHdce and Toriiatellinidce are remnants of this fauna, 

 which was doubtless once nearly or quite world wide, and probably attained 

 its acme in Palaeozoic time. The Heterurethra [Sitccineidcc, etc.) doubt- 

 less existed in this early fauna, as well as the Aulacopoda [Eudodontidce 

 are known from the Carboniferous), and the Heliciuida:. Of the fresh- 

 water forms probably represented in this fauna, we may mention the 

 ancestral stock of fresh-water pearly mussels, the Cyrenacea, the ancestral 

 Melanopsidcc and Melaniidcc, and the Lymuceidcc. With the rise of the 

 Sigmurethrous snails, the land-snails of this primitive fauna declined in 

 all continental areas. 



One of the most remarkable features of the South American fauna is 

 the extreme scarcity of these primitive Orthurethrous land-snails. This 

 group is represented only by a few Pitpillidce and Fernssacidce, probably 

 derived from Middle America in the Tertiary, and closely related to Antil- 

 lean and Mexican species. 



The origin and early differentiation of the Sigmurethrous land-snails is 

 unknown. At the time of their appearance as fossils, in the late Cretaceous 

 and Eocene, the modern families were already more or less clearly blocked 

 out, so far as they are represented by known fossils. From the evidence 

 at hand, derived from the distribution of the groups in the recent fauna, 

 and as Cretaceous and Tertiary fossils, it appears that the evolution of 

 these families had proceeded during Mesozoic time in two chief areas, for 

 which we may use terms proposed in another connection by Dr. Theodore 

 Gill. 



II. Ccoiogccic or noytJiern fauna, occupying old land areas in North 

 America, Asia and Europe, — what is now the Holarctic and part of the 

 Oriental realm, with part of the Neotropical (the Antillean-Central Amer- 

 ican continent). Leading families evolved on this area or areas follow;'" 



^ See Manual of Conchology, XX, Introduction. 



' The groups of low type continued or derived from the preceding fauna are not included. 



