XI] THE SHELLS OF CEPHALOPODS 839 



in connection with the early embryonic shell. In form the Anap- 

 tychus, or the pair of conjoined Aptychi, shew an upper and a lower 

 border, the latter strongly convex, the former sometimes shghtly 

 concave, sometimes shghtly convex, and usually shewing a median 

 projection or slightly developed rostrum. From this rostral 

 border the curves of growth start, and course round parallel to, 

 finally constituting, the convex border. It is this convex border 

 which fits into the free margin of the mouth of the Ammonite's 

 shell, while the other is appHed to and overlaps the preceding whorl 

 of the spire. Now this relationship is precisely what we should 

 expect, were we to imagine as our starting-point a shell similar to 

 that of Hyalaea : in which however the dorsal part of the split cone 

 had become separate from the ventral half, had remained fiat, and 

 had grown comparatively slowly, while at the same time it kept 

 slipping forward over the growing and coiUng spire into which the 

 ventral half of the original shell develops*. In short, I think there 

 is reason to believe, or at least to suspect, that we have in the shell 

 and Aptychus of the Ammonites, two portions of a once united 

 structure; of which other Cephalopods retain not both parts but 

 only one or other, one as the ventrally situated shell of Nautilus, 

 the other as the dorsally plated shell for example of Sepia or of 

 Spirula. 



In the case of the bivalv-e shells of the Lamellibranchs or of the 

 Brachiopods, we have to deal w^ith 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 forin 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 an equiangular 

 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 



* The case of Terebratula or of Gryphaea would be closely analogous, if the smaller 

 valve were less closely connected and co- articulated with the larger. 



