Trochus niloticus and Trochus maximus. 99 



niloticus are almost as broad, regular, and few as those of Tr. 

 maximus : such a specimen of Tr. niloticus is figured by Chem- 

 nitz. The measurements of the largest and of the smallest 

 specimens, which may be supposed to be full-grown, among 

 those exhibited in the Berlin Museum are as follows, the height 

 being measured from the top to the undermost part of the pillar 

 side of the aperture : — 



Tr. maximus: height 130, breadth 130; height 91, breadth 91 millims. 

 Tr. niloticus : „ 112, „ 124; „ 61, „ G7 „ 



The last measurements are taken from an extraordinarily dwarf 

 specimen, the thickened pillar-lip and convex basis of which 

 prove it to be full-grown. 



In almost every full-grown specimen of Tr. niloticus the 

 uppermost whorls are much worn and often wholly destroyed ; 

 this circumstance has probably hitherto formed the chief diffi- 

 culty in recognizing the very young specimens as belonging to 

 the same species. I am compelled by it also to number the 

 whorls in a retrograde manner, from the last upwards. The two 

 vv'horls before the last in Tr. niloticus are, as above stated, 

 even, smooth, and marbled ; they agree in these respects, as 

 well as in size, exactly with the last two whorls of Tr. marmoratus 

 as figured by Chemnitz (tigs. 1606, 1607) ; and on placing several 

 specimens in the Berlin Museum agreeing with those figures 

 side by side with the full-grown niloticus, I feel quite convinced 

 that they are nothing but the same species in a younger state, 

 wanting the last whorl. The base of one of these specimens is 

 as smooth as that of the full-grown niloticus ; the base of an- 

 other exhibits distinct traces of spiral grooves ; in all, however, 

 it is neither convex nor concave, but as even as the spiral 

 growth allows it to be. The second whorl before the last in 

 Tr. marmoratus exhibits a sculpture of large plaits or tubercles 

 near the lower edge, and two or three rows of small grains above 

 them, sometimes uniting themselves into oblique plaits, vanish- 

 ing gradually towards the upper whorls ; therefore the third 

 before the last in the full-grown niloticus must have the same 

 sculpture; but this is just within reach of the apical destruc- 

 tion, so as to show only few and less distinct traces of the sculp- 

 ture in question. Moreover there are in the Berlin Museum 

 smaller specimens agreeing perfectly with Gmelin's Trochus spi- 

 nosus, Chemnitz, fig. 1611, in size, shape, and sculpture, which, 

 on comparison with those named marmoratus, prove evidently to 

 be the young of the same — that is, the very young niloticus, 

 requiring still three or four whorls for its full age, and therefore 

 sculptured on all the whorls which have been as yet formed. 

 In these specimens, also, the base is even and slightly grooved 

 in a spiral direction. 



7* 



