MASON : VARIATION IN THE SHELLS OF THE MOLLUSCA. 337 



In most cases the form of the shell accurately reproduces 

 the form of the mantle, and every fold or projection on the 

 mantle is shown in the sculpture and appendages of the shell. 

 To this rule, however, there are exceptions, where all the 

 processes of the mantle do not secrete shelly coverings, as the 

 Cerithia and Oriental Melanice have delicately digitated mantle 

 margins which are thrown back over the margin of the shell, 

 while in others, as in Marginella and Cyprcece, the lateral lobes 

 of the mantle nearly or completely cover the outside of the 

 shell, which is never covered with epidermis and has therefore 

 that highly polished surface and lustre which give to these shells 

 so brilliant an appearance in our collections. Any injury to the 

 mantle may, therefore, produce a deformity of the shell. In 

 some species the processes on the edge of the mantle seem to 

 be of intermittent growth, and wither away periodically • while 

 present they are protected by a deposition of shelly matter, and 

 in this way the spines and projections found on adult speci- 

 mens are formed. The spines on different specimens of the 

 same species may vary greatly in delicacy and strength, as is 

 shown in Cardmm aciileatum and its variety oimta ; in this 

 variety the spines assume a beautifully arched form. 



The colour of the shell, however, is not an index of the 

 colour of the tenant. The shell may be of the plainest appear- 

 ance while its habitant may be a gorgeous creature, as in the 

 European CyprcBa — a plain white shell is the home of an animal 

 with a dark vermillion proboscis, yellowish-red tentacula spotted 

 with yellow, with the upper part of the foot streaked longitudin- 

 ally by yellow and brown, and with the mantle greenish-brown 

 edged with brownish-red. The colouring matter of the shell is 

 confined to the outer layer, the inner laminge being often 

 nacreous or pearly, as may be seen in Unio pictoriim, when the 

 outer layer has been removed, and is produced by small miliary 

 glands on the edge of the collar of the mantle, the patterns 

 being due to the arrangement of these pigment secreting glands. 

 The object of these brilliant patterns is somewhat mysterious in 



