THE MODIFICATION OF GENERIC CHARACTERS. 29 1 



If wc consider the actual total number of generic types 

 for the whole suborder, and some of the later of these genera 

 are based upon very slight modification of characters, we find 

 76 in all. The rate of their initiation is: Ord. 2, Sil. 20, 

 Dev. 16, Carb. 10, Trias. 16, Jur. 2 ; or by the time of the 

 second stage in which any of the suborders are known one 

 quarter of the total generic differentiation had taken place, 

 and differentiation did not cease till six eras had been passed 

 and the suborders became extinct. 



Representing these facts in whatever way we may, they 

 are positive in testifying to a rapid and early expression of 

 the differences in structure which have served as the means of 

 distinguishing difTerent families, subfamilies, and genera ; and 

 a close inspection of the figures seems to indicate that in 

 proportion to the higher taxonomic rank of the characters, 

 the earlier or more rapid was their initiation. 



General Law of Rate of Initiation of Generic Characters. — In 

 general terms, the scientific fact here noted, irrespective of 

 any theoretical explanation, is that, relative to the known 

 geological range of species of the Helicopegmata, the grander 

 differences in structure were very early to appear, and that 

 the progress of differentiation after this early stage was largely 

 in respect of varietal and specific characters and proportion- 

 ally small in characters of higher rank. 



The Life-period of Genera and the Initiation of a New Genus. 

 — We have now examined some of the laws of genera as ex- 

 hibited in the case of the Helicopegmata. The characters 

 which are found to be of generic value, such as the particular 

 structure of the calcareous framework supporting the brachial 

 arms, have a definite history. Examining all the known 

 Biachiopods, from the beginning of geologic time to the pres- 

 ent, it is found that the structural characters peculiar to this 

 suborder are confined to the time extending from the Lower 

 Silurian to the Triassic or Jurassic era. As a particular ex- 

 ample, for instance, the arrangement of the brachidium char- 

 acteristic of the genus Meristella (see Fig. 81, 7, with the 

 complex loop forming two lateral rings, and the cone of the 

 spirals pointing to the lateral margin of the shell, as in Fig. 

 80, a) begins in the Silurian, and is never seen after the Devo- 



