608 MR. H. F. BLANFORD ON THE DESCRIBED FORMS OF TANALIi^. 



P. Bengalensis), at other times almost entirely concealed. Between these extreme mo- 

 difications every intermediate degree of exsertion may be met with ; and in specimens 

 collected on the same spot (so far as my own experience goes) some variation is always 

 perceptible in this respect. The smaller varieties have, as a rule, the highest spires ; at 

 least I have seen no Paludiniform examples of the larger forms; and the nodulose 

 varieties are less variable in the spire than those with simpler ornamentation, though even 

 the most spiny forms exhibit considerable differences. 



In some specimens of a nearly smooth variety from a feeder of the Kelany Ganga be- 

 tween Ambegammoa and Kitoolgalle, the upper whorls are exposed to considerably below 

 the swell of the whorl ; but from the same spot I have specimens in which the suture coin- 

 cides with the middle of the upper whorl, and others of intermediate degrees of inclination. 



Some specimens which I received from the Orobokka, in the Southern Province, exhibit 

 a similar degree of variability. 



The following comparative measurements, in inches and lines, of specimens of spiny, 

 ribbed, and large and small smooth forms exhibit the extremes of variation I have met with. 



The above measurements can only be taken as roughly indicative of the exsertion of the 

 spire, as the ratio of height to diameter varies also with the dilatation of the shell, and 

 the former is but little affected by a slight exposure of the upper surface of the whorls. 

 Series III. a and III. h of the Plate illustrate the variability of the spu-e in large smooth, 

 and ribbed forms, respectively. 



4. Size of the Shell. 



In no respect do specimens of Tanalia vary more than in size. In this character, 

 again, all the varieties (classed according to sculpture) exhibit a great range of difference, 

 as is shown by the following four series of measurements. The specimens of the first are 

 from the Mahavelli Ganga, illustrating the variability of full-grown sheUs from the same 

 stream ; those of the other three series are respectively spiny, ribbed, and smooth or faintly 

 ribbed forms, selected from various localities. (The measurements are in inches and lines.) 



