340 MASON : VARIATION IN THE SHELLS OF THE MOLLUSCA. 



they may have sharply defined or serrated margins, and they 

 may be represented by interrupted streaks or dots. The fact 

 tliat the growth of the sliell is spiral tends to produce a banded 

 pattern in the markings in univalve shells. In the bivalves, 

 each valve of these being a flattened spiral, the tendency of the 

 colour markings is to form streaks running from the hinge to 

 the periphery of the shell. 



I now come to the subject of the so-called sinistral and 

 dextrorsal varieties or monstrosities. The direction of the 

 spiral is easily observed in the Gastropoda, but it is not so easy 

 to recognise the fact that in the Lamellibranchs one shell, viz., 

 that to the left when the animal is walking, is a dextral and the 

 opposite one a sinistral spiral. In a few cases the reversal of 

 this rule has been observed, as in two specimens of Lucina 

 childreni in the Tankerville collection, and it may be much more 

 frequent than is generally supposed. Among the univalves, the 

 vast majority have a dextral spire, viz., one turning from the 

 left to the right, and which have in consequence the mantle on 

 the right side of the axis ; while the species like the Clausilice 

 have the spire normally sinistral. That this is not a character 

 of high antiquity is proved by the fact that there may be species 

 in the same genus normally dextral and others sinistral, as 

 among our British Cerithia, reticulahiin and perversinn. All 

 shells may have the spire reversed, and in the case of Fusus 

 antiquus there is the remarkable fact that it is possible to trace 

 the change from a normally sinistral to a normally dextral shell, 

 the former being found in the inhabitants of the seas in which 

 the crag deposits were formed, and the latter in the seas on our 

 own shores. In this case it is not very infrequent to find both 

 dextral and sinistral forms living together, doubtless the results 

 of inheritance. It might be supposed that the numbers of 

 specimens of sinistral Fusi and Buccina found in collections are 

 due to the large number of specimens collected for food, but in 

 LUtorina litto/ea, which is literally collected in millions for the 

 same purpose, only three sinistral specimens are known — two 



J.C, vii., Jan. 1894. 



