BY C. HEDLEY. 511 



Mr. Snter in New Zealand, which probably represent the fry of 

 an undescribed species, I am enabled to detail the process. My 

 friend supposes that in unfavourable circumstances a septum is 

 never formed, a view which his American experiences had already 

 suggested to Gibbons. If this be the case, and Gundlaclda some- 

 times continues to regularly enlarge the ancyliform shell, then 

 only an anatomical examination could distinguish between the 

 genera ; and although several supposed species have been named 

 and, more or less adequately, described as Australian, yet this 

 hypothesis would require proof of the existence of Ancylus in 

 Australia. 



The first deviation shown by young Gundlachia from Ancylus 

 consists of a fold appearing at the })osterior end of the aperture 

 (fig. 13). No increase occurs round the rim of the ancyliform 

 shell until the fold is built into a septum flooring half or two- 

 thirds of the original shell. This septum is flat and grows 

 asymmetrically, the right margin advancing before the left. At 

 this stage the shell has much resemblance to a spectacle-case, and 

 has been well figured by PfeifiTer. Vigorous growth now occurs ; 

 in front, but in an altered plane, the margin of the ancyliform 

 shell is continued outwards, behind, the shell is spread beneath the 

 septum floor to form the roof of the secondary shell, then leaving 

 the septum it is abruptly bent downwards. A slight inclination 

 to spiral growth is shown by the increase on the right exceeding 

 that on the left. 



Stimpson suggests "that the Gundlachia commences its life as 

 an Ancylus^ ... it passes the first summer and autumn of 

 its existence in this smaller shell, and that the septum, which 

 afterwards partially closes its aperture, is formed during the 

 period of inaction which ensues during the winter. This septum 

 would in some degree serve as a protection to the mollusc during 

 this period, in the same way as the epiphragm of the Helices. In 

 the following spring — the period of greatest activity in growth 

 with all the fresh-water Pulmonates — the animal throws forth its 

 newer and larger shell, retaining the older one on its back for the 

 protection of its more tender viscera." 



