66 THE NAUTILUS. 
mania, 1878, p. 24): ‘In the young state the shell is simple and re- 
sembles the common Ancylus in the same neighborhood.” 
From the fact that Ancylus woodsii (op. cit. p. 23) is omitted by 
its author from his last catalogue (op. cit. 1890, p. 145) I infer that 
he now considers that name to be a synonym, and further that he 
considers it a synonym of G. petterdi. If so, it is a matter for re- 
gret that Mr. Johnston has withdrawn his species in a manner to 
confuse a student of his writings. 
The published figures of the juvenile shell only represent the 
stage at which the septum is completed and the secondary growth 
is about to occur. Thanks to a series of specimens collected by 
Mr. Suter in New Zealand, which probably represent the fry of an 
undescribed species, I am enabled to detail the process. My friend 
supposes that in unfavorable circumstances a septum is never 
formed, a view which his American experience had already sug- 
gested to Gibbons. If this be the ease, and Gundlachia sometimes 
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 Ancy/us in Australia. 
The first deviation shown by young Gundlachia from Ancylus 
consists of a fold appearing at the posterior end of the aperture, 
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 Pfeiffer. Vigorous growth now occurs; in front, but in an 
altered plane, the margin of the ancyliform shell is continued out- 
wards, behind, the shell is spread beneath the septum floor to form 
the roof of the secondary shell, then leaving the septum it is ab- 
ruptly 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 Gund/achia 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 de- 
gree, serve as a protection to the molluse during this period, in the 
