20 SUPPLEMENT TO THE CRAG MOLLUSCA. 



by the late Col. Alexander, but, unfortunately, it has no special locality attached. It is 

 undoubtedly a Crag shell, and from its appearance it looks like one of the Fluvio-marine 

 specimens of Bramerton. The sinistral form of T. antiquus, occasionally found at the 

 present day in the British seas, is simply striated, and not carinated, corresponding in 

 that respect with the shell so abundant at Walton-on-the-Naze. 



In the ' Crag Mollusca,' vol. i, p. 45, 1 have expressed an opinion that this left-handed 

 striated whelk was, in British seas, probably the original form, in opposition to the 

 general statement of conchologists that it is merely a variety, in consequence of the dif- 

 ference displayed from the common right-handed shell of the present day. 



The great majority of shelled univalved Mollusca have the volutions turned in a dextral 

 direction, that is, from left to right, but whether the original inflexion was given to the right 

 or to the left we do not know, or why they should have taken the one in preference to the 

 other. Among the Cephalopoda, the oldest known form is the straight one, as in 

 Orthoceras. The bend from this seems to have been first in a vertical direction, such as 

 Phragmoceras or Toccoceras. The deviation from that vertical direction was, I conjec- 

 ture, due to the partial atrophy of the organs on one side, from a slightly altered position 

 of the heart, until the highly oblique growth of the Turrilite was reached. 



Fusus sinistrorsus, Lam., is now an inhabitant of the Mediterranean Sea, and it is also 

 a fossil in the newer Tertiaries of Sicily, and this may be a descendant of the older form 

 of the Walton Crag sea. I can perceive no difference sufficient to constitute the 

 Mediterranean shell a different species from the Crag fossil. It is, therefore, somewhat 

 remarkable that in Trop/ion antiquus this sinistral form should be the only one found in 

 the Crag of Belgium, appearing there in the middle and upper beds, both dextral and 

 sinistral forms being unknown in the lower, as they are also in the Cor. Crag of this 

 country, thus apparently showing that the dextral form of this shell was of more modern 

 origin than the sinistral, and that it had not appeared during the earlier part of the Red 

 Crag. The left-handed " Almond Whelk " is the only form of this variable species which 

 is found in the Red Crag of Walton-on-the-Naze (the whole fauna of which locality is, 

 in my opinion, clearly older than that of any other part of the Red Crag) ; for while I 

 have seen thousands of the sinistral shell from this locality, I have never met with one of 

 the dextral form there, or seen a specimen of it in the possession of any collector from this 

 place. In the rest of the Red Crag the dextral and sinistral forms of the striated shell 

 seem present in about equal proportions, and the same thing occurs in the Fluvio-marine 

 Crag, in the Chillesford bed passim, and in the Lower Glacial sands of Belaugh, Rackheath, 

 and Weybourne. In the Middle Glacial sands, however, the only trace of the sinistral form 

 that has occurred is the pullus of some sinistral Trojjhon, which is probably contrarius ; 

 while several perfect young specimens, and one full grown, as well as numerous fragments 

 of the columella of the dextral shell, have occurred. It would thus seem that the life of 

 this species, so far as the seas of Britain and Belgium reveal it, exhibits the curious 

 feature of having begun exclusively left-handed, then to have varied by the birth of 



