XI] 



THE SHELLS OF PTEilOPODS 



575 



strengthened and stiiTened by its midrib, is nearly straight, while 

 the curvature of the other is well displayed. But the case will 

 be materially altered and simplified if growth be arrested or 

 retarded in either half of the shell. Suppose for instance that 

 the dorsal valve grew so slowly that after a while, in comparison 

 with the other, we might speak of it as being absent altogether : 

 or suppose that it merely became so reduced in relative size as to 

 form no impediment to the continued growth of the ventral one ; 

 the latter would continue to grow in the direction of its natural 

 curvature, and would end by forming a complete and coiled 

 logarithmic spiral. It would be precisely analogous to the spiral 

 shell of Nautilus, and, in regard to its ventral position, concave 

 towards the dorsal side, it would even deserve to be called directly 



Fig. 301. Pteropod shells, from the side: (1) Cleodora ciispidala; (2) Hyalaea 

 longirostris ; (3) H. trispinosa. (After Boas.) 



homologous with it. Suppose, on the other hand, that the ventral 

 valve were to be greatly reduced, and even to disappear, the 

 dorsal valve would then pursue its unopposed growth ; and, were 

 it to be markedly curved, it would come to form a logarithmic 

 spiral, concave towards the ventral side, as is the case in the shell 

 of Spirula*. Were the dorsal valve to be destitute of any marked 

 curvature (or in other words, to have but a low spiral angle), it 

 would form a simple plate, as in the shells of Sepia or Loligo. In- 

 deed, in the shells of these latter, and especially in that of Sepia, 

 we seem to recognise a manifest resemblance to the dorsal plate of 

 the Pteropod shell, as we have it (e.g.) in Cleodora or Hyalaea ; 



* Cf. Owen, "These shells [Nautilus and Ammonites] are revolutely spiral or 

 coiled over the back of the animal, not involute like Spirula": Palneontolcgy, 

 1861, p. 97; cf. Mem. on the Pearly Nautilvs, 1832; also P.Z.S. 1878, p. 955. 



