102 GENERAL EVOLUTION". 



I append two homologous series, represented by the Nautilea 

 and the Ammonites of the Tetrabranchiate Cephalopoda, which 

 are distinguished, the first by the simple septa and the siphon 

 central or marginal ventral ; and the second by the complex and 

 folded septa and siphon central or marginal dorsal. The parallel- 

 isms have been noted by Barrande, Bronn, and many concholo- 

 gisfcs, who can furnish a much more full table than the following, 

 from the most recent sources : 



Nautili. Ammonites. 



A. The shell straight, unwound. 



Orthoceras. Baculites. 



B. The shell more or less curved or wound, 

 c. Simply curved. 



Cyrtoceras, ) Toxoceras. 



Phragmoceras, ) 



aa. A more or less straight portion, folded on the remainder. 

 j3. Folded poition in close contact with remainder. 

 Ascoceras. Ptychoceras. 



/3/3. Folded portions not in contact. 

 V ? Hamites. 



aaa. One extremity spirally wound, the volutions not in contact. 

 /?. Extremity of the shell prolonged beyond the wound portion. 

 Lituites. Ancyloceras. 



)8/3. Extremity not prolonged in a line. 

 7 The spiral flat. 

 Gyroceras. Crioceras. 



yy. The spiral elevated (heliciform), 

 Trochoeerus. Turrilites. 



aaaa. Spiral turns of the shell in contact. 



p. Er.tremity prolonged in line beyond the spiral. 

 * * Scaphites. 



)S/3. Extremity not prolonged beyond spiral. 

 Nautilus. Amrhonites. 



We may now consider the question of the origin of these higher 

 groups. In the first place, we must lay down the proposition that 

 the characters which constitute groups '' higher ''^ iii the compari- 

 son of ranh (we do not of course mean higher in the same line, as 

 we say higher genus in a family, or higher order in a class) are 

 such solely from their bei7ig more comprehensivs, or i^esent 

 throughout a greater range of species. 



What is true, therefore, in respect to characters of genera, is 

 likely to be true in respect to characters of higher groups, such 

 as we have been considering in the preceding pages. Believing, 

 then, that a new genus has been established by the transition of a 

 number of species of a preceding genus in-order, without neces- 

 sary loss of specific characters, I think the same process may have 

 established the suborders and orders in question. That is, that a 

 large numiber of genera have near the same time, in past or pres- 



