98 Dr. E. von Martens on the different Ages of 
illustration of niloticus. HH. & A. Adams, ‘ Genera of Molluscous 
Animals,’ i. p. 418, admit three species—Tr. maximus, niloticus, 
and spinosus ; under the last name they perhaps also comprehend 
Lamarck’s marmoratus, on the authority of Philippi, who unites 
both into one species. 
In the zoological museum of Berlin there are at present 
twenty-seven specimens which I have been able to examine for 
this purpose —five full-grown 77. niloticus, four full-grown Tr. 
maximus, twelve which I presume to be young ages of niloticus, 
and six which I think to be the young of 7. maximus. 
The differences of the two species, when full-grown, prove 
to be very striking; they are the following :-— 
1. Tr.maximus is as high as broad, niloticus is broader thanhigh. 
2. In Tr. maximus the two whorls before the last exhibit a 
distinct sculpture of large oblique folds; in 77. niloticus they are 
as smooth as the last itself. 
3. In Tr. maximus the last whorl has its upper surface 
slightly curved, in the same manner as the preceding ones; in 
Tr. niloticus it is remarkably concave, the peripheral margin 
being very swollen and projecting, whereas the surface of the 
preceding whorls is even. 
4. The base or under surface of Tr. maximus is concave, that. 
of niloticus convex. 
5. The same part exhibits in 77. maximus about twelve spiral 
grooves, whilst in 77. niloticus it is smooth. 
6. The markings also are somewhat different : in 7. maximus 
they consist of distinct large purple rays, which become somewhat 
more numerous on the last whorl; in 77. naloticus the rays are 
much more numerous, narrower, and variously bent in the last 
whorl; on the preceding ones they are interrupted and inter- 
mingled one with the other so as to form a repeatedly rather 
marbled design. The base, on the contrary, exhibits in 77>. 
maximus small linear spots situated between the grooves, and, 
therefore in the direction of the spiral—in 77. niloticus narrow 
radiating lines, which are variously bent and interrupted. 
In both the markings of the base are of a lighter red than those 
of the upper surface, and in both the purple colour of the latter 
is sometimes partially replaced by a rather dark green. I sup- 
pose the latter to be the original colour, which may be changed 
into red in the dead shell, or the dead portions of it, by the in- 
fluence of light, just as the same change of colour happens in 
Neritina fluviatilis by long exposure to the light of the sun. 
Both species are subject to individual variations within rather 
narrow limits. The dilatation of the last whorl of 7. niloticus 
and the convexity of its base especially are different in degree 
in different specimens; and the rays of some specimens of Tr, 
