146 Memoirs of the Indian Museum. [Vol. VII, 



numerary varix, measures 115 x 89 mm. Nevertheless, as there is never more than one 

 pre-apertural varix, and as, when present, it is invariably situated close to the final 

 aperture from which it is separated by an angular distance of from 30 to 8o°, it is 

 evident that the shells exhibiting this character are adult, or, at least, have completed 

 the growth of which they were capable. In certain species of Dolium, such as 

 D. fasciatum, D. zonatum, D. tessellatum, D. crosseanum, the internal, and in some 

 cases also the external thickening of the outer lip is invariably or almost invariably 

 present in every specimen, quite irrespective of size, and, as it is quite inadmissible 

 that all these specimens should have reached the termination of their growth, it is 

 clear that, as in the case of many other gastropods, the animal is able to absorb the 

 apertural structures at each successive phase of growth. The varicose specimens of 

 Dolium fasciatum at present under consideration represent, therefore, individuals 

 which, on approaching the final term of their growth, have lost the power of resorp- 

 tion ; and, as the average size of these specimens is below that of the normally full- 

 grown shell in which the pre-apertural varix is absent, they probably represent 

 individuals the vitality of which has been impaired through insufficient nutriment or 

 some other cause. 1 It seems evident that the majority of the specimens of Dolium 

 fasciatum reach their final stage of growth without leaving, on the body-whorl, any 

 trace of this pre-apertural varix. 



Nevertheless, the presence of this structure, though not constant, is of consider- 

 able interest as the first instance hitherto recorded of a feature generally absent from 

 the Doliidee, but characteristic of many Cassididœ ; especially if we take into con- 

 sideration the close relationship between Dolium fasciatum and the forms that have 

 been referred to the sub-genus Eudolium, the resemblance of which to the Cassididse 

 has frequently been commented on, and is sometimes so pronounced that certain 

 forms of this group have on several occasions been erroneously referred to Cassi- 

 daria. 



The Indian Museum collections contain one more example of a Dolium exhibiting 

 a supernumerary varix (text-fig., p. 147). This is a handsome specimen of Dolium 

 zonatum from Hong-Kong, measuring no less than 142 x 112 mm. 2 The supernumer- 

 &vy varix is situated at an angular interval of about 55 ° from the outer lip whose edge 

 has not yet received its final thickening, the growth of the shell being, apparently, still 

 incomplete. In this species, the apertural thickening does not affect both the external 

 and internal borders of the aperture as in Dolium fasciatum, but is developed only 

 internally. Consequently, the supernumerary varix, in the present instance, is not 

 conspicuous externally where it is indicated only by a slight swelling of the surface, 

 bordered, on its forward side, by a linear groove. Internally it is very strongly 

 developed and exhibits all the typical features that characterise the labrum of a full- 

 grown specimen. 



1 Compare Dr. Annandale's remarks concerning the varix of Hydrobioides nassa from the Inlè Lake {Bec Ind. Mu». , 

 Vol. XIV, pp. 172 173), in which, however, the varix, as in certain other Gastropoda, is a constant adult character. 



2 Tt has not been possible to prepare a photographic reproduction of this fine specimen which came to notice only 

 after the plates illustrating Dolium fascintwn had already been prepared. 



