336 GEOLOGICAL BIOLOGY. 



and in Ammonites forward, and possesses the same structure with the 

 septum. Ordinarily, the siphonal collar has only short length, and forms 

 in front and behind the septum a sheath in the form of a band or collar 

 about the siphon; but, sometimes, they pass from o.ie septum to the other 

 and form there a close continuous tube, or they have the form of an open 

 funnel, slightly contracted behind, and prolonged to the next following sep- 

 tum, or even go beyond it, thus implanting themselves one within another 

 (telescoping, Endoceras). The siphon is found in the median plane of the 

 shell, and it is only exceptionally that it deviates a little from this plane. 

 In this plane its position vacillates from the external side to the internal 

 side in the different genera and the different species. 



In the Ammonitidae it is constantly on the external side of the shell. 

 In the Nautilidae its position does not remain constant in one and the 

 same genus : it may be external, internal, central, or intermediary. 



Numerical Rate of Differentiation expressed in Terms of the In- 

 itiation of New Genera. — A study of the statistics of classifi- 

 cation in relation to time will exhibit in this, as it has in 

 previous cases, the grand features of the historical differenti- 

 ation of the cephalopods. 



First, we may consider what are the conclusions to be 

 drawn from the succession of new genera as to the rate and 

 order of the differentiations of the class Cephalopoda. 



The classification itself is expressive of differentiation, as 

 has been already observed. The division of the class into 

 two orders is expressive of a very marked differentiation in 

 structure. The genus is a group of organisms with the same 

 ordinal and family structure, but exhibiting some particular 

 characters, such as shape, relative size of parts, or special de- 

 velopment of some part, which are the same for several dif- 

 ferent species ; hence we recognize the number of genera to 

 be a numerical expression of the amount of differentiation 

 attained in the family at any particular period of time, and 

 the greater the number of genera in a particular family, at a 

 particular time, the greater is the amount of differentiation 

 expressed in the family-history at that period, and the 

 number of genera beginning or living in each period becomes 

 a rough indication of the rate of expansion or evolution of 

 the race under consideration. The total number of genera 

 in the order Tetrabranchiata is 123 (Zittel). Two grand 

 subdivisions of subordinal rank are made, including, respec- 

 tively, Nautiloidea 29 genera, and Ammonoidea 94 genera. 

 28 genera of the 29 Nautiloidea had appeared in the Silurian. 



