98 Dr. E. von Martens on the different Ages of 



illustration of niloticus. H. & A. Adams, ' Genera of Molluscous 

 Animals/ i. p. 413, 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 7/-. 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 Tr. 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 Tr. 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 Tr. maximus about twelve spiral 

 grooves, whilst in Tr. niloticus it is smooth. 



6. The markings also are somewhat different : in Tr. maximus 

 they consist of distinct large purple rays, which become somewhat 

 more numerous on the last whorl ; in Tr. niloticus the rays are 

 much moi'c 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 Tr. 

 maximus small linear spots situated between the grooves, and 

 therefore in the direction of the spiral — in Tr. 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 Tr. niloticus 

 and the convexity of its base especially are different in degree 

 in different specimens j and the rays of some specimens of Tr. 



