PARALLELISM. 183 



has been previously stated, the first is rounded and 

 smooth, with simple septa \ the second tuberculated, 

 and the septa more complicated ; the third was the 

 only one in which the septa, form, and ornamentation 

 simultaneously attained the climax of individual com- 

 plication; the fourth, when amounting to anything 

 more important than the loss of a few ornaments, was 

 marked by a retrogression of the whorl to a more tabu- 

 lar aspect, and by the partial degradation of the septa. " 



I am indebted to Professor Hyatt for the following 

 more detailed account of the results of his researches 

 in this interesting field. The evidence as to the na- 

 ture of evolution derived from the Cephalopoda is more 

 complete than that obtained from any other source. 



"Every group of nautiloids passes through, during 

 its evolution in time, either a part or the whole of a 

 certain series of changes. These modifications con- 

 sist : first, of a straight or nearly straight cone, ortho- 

 ceran j second, a curved cone, cyrtoceran ; third, a 

 coiled cone, gyroceran, which does not come in con- 

 tact at any point; fourth, a coiled cone, nautilian, which 

 does come in contact at the termination of the first vo- 

 lution and then during further growth remains in about 

 the same condition, all of the internal whorls being 

 visible as in a flat coil of rope ; fifth, a coiled cone, 

 involute-nautilian, which also comes in contact like 

 the fourth but then the whorl growing with greater 

 rapidity spreads internally, covering up more or less 

 of the internal volutions sometimes to such an extent 

 that even the centre is concealed from view. The ex- 

 amples which I have myself seen of the fifth kind range 

 from the Silurian to the Nautili of the existing fauna, 

 some being present in every period, and of other kinds, 

 the first, second, and third kinds die out gradually, 



