2 24 SECTION C. INVERTEBRATA. 



them, sinistral specimens alone occur\ in both very abundantly. 

 It is in Pliocene strata of later age, in East Anglia and on the 

 continent, that dextral forms make their first appearance, including 

 not only the typical species Neptunea ayitiqua, but the more northern 

 N. carinata and N. despecta. At lower horizons of the Upper 

 Crag (later than that of Walton), right-handed forms are very rare, 

 but they are increasingly common in the higher zones, while pari 

 passu, left-handed ones, which are abundant in the older, become 

 more, and finally exceedingly scarce in the newer beds, until the 

 relative proportion between them is reversed. 



The facies of the molluscan fauna of the oldest of the Crag 

 deposits is preponderatingly southern, resembling that of the Medi- 

 terranean, or even the Azores at the present day, but as the Glacial 

 period approached, the climate of the Pliocene period gradually 

 became colder, and the Crag basin was invaded from the north by 

 boreal and arctic mollusca, at first in small, but afterwards in 

 increasing numbers ; at the same time the southern species became 

 gradually less abundant, and finally died out altogether, so that 

 eventually the molluscan fauna of the North Sea acquired a 

 decidedly northern character. Now the dextral varieties of 

 Neptunea antiqtia, including its carinated allies, came into the Crag 

 area with the northern shells, becoming abundant as they did, 

 while, as the southern species became less so, and finally disap- 

 peared, Neptunea contraria came to be correspondingly rare. 



The fact that the latter appeared first in the Anglo-Belgian 

 basin led Searles Wood to believe that the progenitor of these 

 different shells may have been left-handed, and that the dextral 

 rather than the sinistral forms should be regarded as varieties. 

 Whether this be so or not, it seems that at the time of their fiirst 

 recorded appearance these mollusks had become separated into 

 distinct groups, which migrated separately, and that the one pene- 

 trated during the Pliocene epoch, and still lives further to the south 

 than the other. 



Comparing these different mollusca, recent and fossil, we find 

 there is a greater general resemblance between the varieties of each 

 group than there is between any of the dextral and sinistral forms. 

 The volutions of the left-handed are, as a rule, more obliquely 

 spiral than those of the right-handed shells, the spire of the former 

 being in consequence more produced, while in the latter the mouth 

 is generally wider, and the canal shorter in proportion. The princi- 

 pal difference between the various forms of each division is in the 

 sculpture. Fig. i is that of a specimen from the Pliocene beds of 

 Sicily- (type of Neptunea sinistrorsa^. The spiral markings by 

 which it is ornamented are close together on the upper whorls, 

 and on the body whorl are divided by a fine thread. Figs. 2 and 3 



^ One right-handed specimen is said to have been found at Walton during the present 

 century. 



- Fig. 1 is that of an immature specimen. 



