MOLLUSCA OF IXDIA. 225 



Benson described a species, /S. schutostelis, also from Moulmeiu, 

 at the same time as he described /S. calias. Stoliczka, on good 

 grounds and with the animals before him, considers this only a 

 variety and due to increased growth of the shell. I give his 

 description of the variety schistostelis (pi. xix. fig. 8) and the very 

 interesting instructive remarks that follow it (pp. 255-57) : — 



" Testa ultimo anfractu multo majore, tenui, pellusido ; apertura 

 semilunari, marginibus tenuissimis ; labro columellari modice obliquo, 

 Levi, supra rejlexo, labro supra paulo Jiexmse producto, ad basin 

 fere recto ; carina umbilicali distincta, tenui, ad aperturam paulo 

 incisa. 



" Diam. maj. spec. max. 17 mm.; diam. min. 14'5 ; axis 8'5 ; 

 alt. testae 11, alt. apert. 8, lat. apert. 9 mm. 



" The species offers a remarkable instance of variation during 

 different stages of growth. As the type I consider the smaller form 

 with a solid shell, the coluraellar lip very oblique and rugose, the 

 outer lip obtuse and internally slightly thickened, and the iimbilical 

 ridge with a deep incision. This type is represented in fig. 7, on 

 plate xix. Small specimens, measuring only 5 mm. in the larger 

 diameter, occur of exactly the same fox'm ; it seems, therefore, that 

 they often attain maturity at an early state. 



" Very commonly, however, it is the case that the shells grow 

 further after they have attained that certain stage of maturity. 

 The increase amounts from one third sometimes to one and a half 

 circuit of a whorl, as indicated in the fig. 8 b. This additional 

 portion of the shell is always thinner than the rest and more trans- 

 parent, the outer lip of the aperture is at the suture less produced 

 on to the penultimate whorl, the coliimellar lip less oblique, thin, 

 smooth, and the umbilical ridge is only slightly incised. In this 

 stage the species was described by Benson as SojyJi. schistostelis, and 

 it is certainly a most marked variety. There can, however, be no 

 doubt that it is only an abnormal growth, for when the terminal 

 half of the last whorl, iudicated in hg. 8 b, is broken away, a typical 

 Sojih. calias of the shape represented in figs. 7, 7 a, 7 b, can be 

 obtained. 



" There appears to be no rule as to the size of the shell at which 

 the abnormal growth begins (or, in other words, at which ^. calias 

 is changing into S. schistostelis), but the latter is locally so constant 

 that very few specimens stop growth at the normal stage, while 

 the abnormal forms are met with in thousands. It is really difficult 

 to decide in such cases whether we ought to call these abnormal 

 forms distinct species or not. But the fact clearly shows how 

 species are developed one out of the other. In this case no one 

 will doubt the propriety of regarding the larger form as an abnormal 

 growth of the smnller one, because the original type can still be 

 traced. But supposing the peristome of the normal shell bad been 

 entirely absorbed, and then the growth proceeded as usually ; in 

 such a case it would be much more difficult, and sometimes quite 

 impossible, to trace the connection of the two forms, which could 

 then with more propriety be acknowledged as two distinct sj)ecies. 



