LIFE ON THE EARTH. 99 



most numerous group, are peculiar to the Oolitic and 

 Cretaceous formations. The recent genera have an 

 ink-bag containing black pigment; this organ is 

 recognized in several of the fossil races, filled with 

 the fossil ink, which retains its good colour, and has 

 been successfully employed in drawings. 



The Polypod genera were (unlike all those pre- 

 viously mentioned except Argonauta) protected by 

 an external shell ; in these it was concamerated, the 

 chambers formed by transverse plates which were 

 pierced by a pipe opening to the last or outer cham- 

 ber, into which the whole or principal part of the 

 animal could be retracted. 



It is usual to rank them in three families Ortho- 

 ceratidae, Nautilidae, Ammonitidse. Of these, the first 

 did not pass 1 the Palaeozoic period ; Ammonitidse did 

 not pass the Mesozoic period ; but Nautilidae, which 

 began in the earliest ages, lived in all the subsequent 

 seas, and still adorn, though not plentifully, the 

 modern ocean. Among the Nautilidae, however, Cly- 

 menia ceased before the close of the Palaeozoic age. 

 The geological range of the Ammonitidae is still more 



1 Unless in the single case of the Triassic beds of Saint Cassian, 

 which contain what seem to be Orthoceratites, and also very pecu- 

 liar Ammonitidse having the numerous gradually diminishing 

 lobes of Ceratites, with the highly ramified sutures of the Oolitic 

 Ammonites. 



II 2 



