228 THE TRANSFORMATIONS OF THE ANIMAL WORLD 



by their exterior form from certain Eupsammias 

 with porous partitions. In the Gastropod molluscs 

 the spiral coil, or, on the other hand, the uncoiling 

 of the shell is produced by parallel processes in 

 groups very dissimilar as regards their internal 

 organs. Thus the form of shell in a simple sur- 

 based cone, with the hardly projecting head of 

 which the Limpets which fix themselves on the 

 rocks of our coasts offer the best-known type, 

 manifests itself again in the Prosobranchs in the 

 families of the Fissurellids (Emarginula, Parmo- 

 phorus), of the Neriditae (Navicella), of the Tecturidse 

 (Patella, Helcion), of the Capulidae (Gapulus, Calyp- 

 trcea), of the terrestrial Pulmonates (Ancylus), or 

 Thalassophilse (Siphonaria) ; and, lastly, among 

 the Opisthobranchs (Umbrella). It is true that a 

 naturalist with a little experience will not hesitate 

 to recognize by this or that morphological detail to 

 which of these numerous patelloid groups should be 

 ascribed any shell submitted to him for examination. 

 But it is especially in the fossil Cephalopods that 

 the ingenuity of palaeontologists has been most suc- 

 cessful in discovering facts of convergence. For 

 a long time there have been known, and I have had 

 occasion to dwell on this point, the parallel cases of 

 the uncoiling of the shell among the Nautilidaa and 

 the Ammonites. In these two groups there can 

 be established a series of forms going from a straight 

 shell to a closely coiled one like that of the Nautilus, 

 by the intermediary of forms more and more in- 

 curved or with loose coiling. But, here at least, 

 it will always be possible to distinguish the straight 

 Nautilus Orthoceras, with partitions added end to 



