472 Pi'oceedings of the Royal Fhysical Society. 



species are exceedingly uncommon, and mostly of large size. 

 During the Paleozoic and Mesozoic it abounds in the 

 deposits, and its ornamentation is very various. It appears, 

 therefore, to differ in these respects from most other types 

 showing great persistence in time. On the other hand, the 

 genera and species of the family Nerineidse, with their extra- 

 ordinary specialisation of the columella, have very short 

 ranges, and the whole group is entirely Mesozoic. 



The Nautiloid type has shown great persistence ever since 

 early Cambrian times, and with very little variation in the 

 character of the shell. It contrasts well with the extra- 

 ordinary variety of forms developed, as specialised branches 

 of the Nautiloid stock during its period of dominance in the 

 Palaeozoic. Even amongst these the simple genus Orthoceras 

 has the longest range, extending from the Cambrian to the 

 Trias ; and the members of the family Orthoceratidse, as a 

 whole, have long ranges in comparison with the more 

 specialised groups. The extraordinary variety of specialisa- 

 tion attained to in the Ammonites is as remarkable as the 

 exceedingly limited range of time shown by the different 

 genera and species, and the enormous numbers in which they 

 must have existed. There is a peculiar tendency in many of 

 the groups of fossil Cephalopoda to show, during their decline 

 preceding extinction, forms which may be compared to senile 

 or gerontic forms of less specialised species and genera. The 

 Ammonites apparently underwent complete extinction on 

 the close of the Mesozoic period, unless we have in the 

 Octopoda that are now existing forms sprung from the same 

 stock, with complete loss of the shell. 



The Belemnites range from the Trias to the Cretaceous. 

 Aulacoceras of the Trias has a very large phragmacone, some- 

 what resembling Orthoceras, and the guard is proportionately 

 small. The typical Belemnites of the Jurassic have a large 

 dactyliform guard, with the phragmacone greatly reduced. 

 During this development they appear to be at their maxi- 

 mum, and abound in the deposits. Whether the Spirtolirostra 

 of the Tertiaries, and Spirula which is still existing, are forms 

 which have persisted owing to not having taken part in such 

 a specialisation of the guard, it would be interesting to know. 



