XI] THE SHELLS OF PTEROPODS 577 



have in the shell and Aptychus of the Ammonites, two portions 

 of a once united structure; of which other Cephalopoda retain 

 not both parts but only one or other, one as the ventrally 

 situated shell of Nautilus, the other as the dorsally placed shell 

 for example of Sepia or of Spirula. 



In the case of the bivalve shells of the Lamellibranchs or of 

 the Brachiopods, we have to deal with a phenomenon precisely 

 analogous to the split and flattened cone of our Pteropods, save 

 only that the primitive cone has been split into two portions, not 

 incompletely as in the Pteropod (Hyalaea), but completely, so 

 as to form two separate valves. Though somewhat greater 

 freedom is given to growth now that the two valves are separate 

 and hinged, yet still the two valves oppose and hamper one 

 another, so that in the longitudinal direction each is capable of 

 only a moderate curvature. This curvature, as we have seen, is 

 recognisable as a logarithmic spiral, but only now and then does 

 the growth of the spiral continue so far as to develop successive 

 coils : as it does in a few symmetrical forms such as Isocardia cor ; 

 and as it does still more conspicuously in a few others, such as 

 Gryphaea and Caprinella, where one of the two valves is stunted, 

 and the growth of the other is (relatively speaking) unopposed. 



Of Septa. 



Before we leave the subject of the molluscan shell, we have 

 still another problem to deal with, in regard to the form and 

 arrangement of the septa which divide up the tubular shell into 

 chambers, in the Nautilus, the Ammonite and their alHes (Fig. 

 304, etc.). 



The existence of septa in a Nautiloid shell may probably be 

 accounted for as follows. We have seen that it is a property of 

 a cone that, while growing by increments at one end only, it 

 conserves its original shape: therefore the animal .within, which 

 (though growing by a different law) also conserves its shape, will 

 continue to fill the shell if it actually fills it to begin with: as 

 does a snail or other Gastropod. But suppose that our mollusc 

 fills a part only of a conical shell (as it does in the case of Nautilus) ; 

 then, unless it alter its shape, it must move upward as it grows in 

 the growing cone, until it come to occupy a space similar in form 



T G. 37 



