160 Records of the Indian Museum. [Vol. XIV, 



due to the absence or presence of minute algae on the surface of the 

 shell. 



In the genera we have as yet considered it has been possible, at any 

 rate in the more conspicuous cases, to trace some one line of evolution, 

 but we have now to consider a genus in which matters are more com- 

 plicated, namely the genus Taia. In some respects Taia is comparable 

 to Hydrohioides, having originated, probably on the Shan Plateau, from 

 a genus with a simple, almost smooth shell, but possessing itself a shell 

 with pronounced and peculiar sculpture. The genus from which it is 

 derived is the almost universally distributed Vivipara. As we shall see 

 presently, this genus has undergone a very similar but quite inde- 

 pendent course of evolution in other parts of the world also. I have 

 thought it most convenient to recognize no less than eleven species of 

 Taia among the living and extinct forms of the Inle, He-Ho and Hsin- 

 Dawng basins, but some of them are closely allied and might doubtless 

 be regarded from a purely taxonomic point of view as races (sub- 

 species) or varieties, rather than distinct species. These forms, whatever 

 we may call them, do not follow a single line of evolution but diverge 

 from one ancestral type in different directions. The ancestral type is 

 represented by a species (T. theohaldi) that still survives but is known 

 also in a fossil state. The relationship of the other ten forms to this 

 species and to one another is shown in a diagrammatic fashion in the 

 figure on the opposite page. Two other species, one of which is still 

 unnamed and very imperfectly known, are found in different parts of 

 the Irrawaddi system. The possible place of one of these (T. noeilingi) 

 is shown in the diagram, but it may have been derived from an 

 unknown form analogous to T. intermedia, or direct from T. naticoides. 



T. theohaldi, if it stood alone, might be accepted as a somewhat 

 abnormal type of the genus Vivipara comparable to V. quadrata 

 (Benson) from China, but it is distinguished from all the species of 

 that genus sensu stricto by the structure of the columellar callus, and 

 from most of the species by the spiral ridges on its shell. These are 

 the two chief generic characters of Taia ; they are less strongly developed 

 in T. theohaldi than in other forms. T. theohaldi has a wide range in 

 the Southern Shan States, and was originally described from " Burma." 

 It is an inhabitant of small streams, and single shells have been 

 found both in the cave deposits of the Hsin-Dawng valley and in the 

 superficial deposits of the He-Ho basin. 



From a geographical and biological point of view the species of Taia 

 that occur on the Shan Plateau fall into four groups. Firstly we have 

 two non-lacustrine species, T. theohaldi and T. naticoides. that have a 

 wide or fairly wide range on the plateau, if not beyond its limits. Then 

 we have three little groups of lacustrine species each of which is, or was, 

 peculiar to a single lake — one to the old He-Ho lake, one to the smaller 

 but probably still older Hsin-Dawng lake, and a third to the existing 

 Inle Lake. All these lakes must have in a sense belonged to the same 

 lake-system, but probably intermigration of the fauna had many 

 obstacles. 



The two surviving widely distributed, non-lacustrine species are 

 certainly among the most primitive in the genus ; to one of them (T. 



