256 F. Stoliczka — Notes on Terrestrial Mullusea. [No. 3, 



eolumellari niodice obliquo, levi, supra renexo, labro supra paulo 

 flexuose producto, ad basin fere recto ; carina umbilicali distincta, 

 tenui, ad aperturam paulo incisa. Diani. maj. spec. max. 17 mm. ; 

 diam. min. 14*5 ; axis S'o ; alt. testee 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 columellar lip very oblique and rugose, 

 the outer lip obtuse and internally slightly thickened, and the 

 umbilical ridge with a deep incision. This type is represented 

 in fig. 7, on plate xix. Small specimens, measuring only 5 mill, 

 in the larger diam., occur of exactly the same form ; it seems, 

 therefore, that they often attain maturity at an early stage. 



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 figure 8 b. This additional portion 

 of the shell is always thinner than the rest, and more transparent, 

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

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

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

 this stage, the species was described by Benson as Soph, shistostelis, 

 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, indicated in fig. 8 b, is broken away, 

 a typical Soph, calias of the shape, represented in fig. 7, 7a, 7b, 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 

 a S, calias is changing into a 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 shews, how species are developed, one out of the 

 other. In this special case no one will doubt the propriety of 

 regarding the larger form as an abnormal growth of the smaller 

 one, because the original type can still be traced. But supposing 



