﻿196 H. S. Williams — Lamellibranchiata and Sjiecies-maJcing. 



" Leiopteria "), while in the figures we find specimens fully 

 as "auriculate" as in several representatives of the other 

 genus, and in the description of half a dozen species of 

 Leptodesma we find the distinct statement that the anterior 

 end is " round." 



In another new genus, for which about a score of species are 

 defined, the writer has seen a series of specimens, picked out of 

 a single block of stone, expressing, by almost imperceptible 

 gradation, almost every character exhibited by the whole range 

 of species figured except difference of size. 



The evil of this species-making habit is only aggravated by 

 the minuteness and carefulness of the observer, and in the 

 study of fossils it is particularly aggravated by the rarity of 

 good specimens and the imperfection of the characters expressed. 

 In its extreme it has ceased to be a classifying of organisms, 

 or even fossils, and has become merely a narration of the dif- 

 ferences exhibited by specimens. 



And when a student seeks to identify new material, the more 

 careful his scrutiny, the more surely is he driven to the idea 

 that specimens which do not fit any of the given descriptions 

 must be new species. So long as the Cuvierian school prevailed, 

 there was some excuse for the practice, but in this age when the 

 belief in the fixity of specific characters and limits is shown by 

 theory and observation to be erroneous, there is no reason for 

 neglecting the element of variability as a factor in the classifica- 

 tion and definition of organisms, whether living or fossil. 



The practice of differentiating species (and genera) by charac- 

 ters which are not clearly discernible on every fair set of the 

 representatives of the species, or by characters which in point 

 of variability among individuals from the same locality exceed 

 the extent of modification separating two species of the same 

 genus, is scarcely defensible, and leads to more confusion and 

 uncertainty than if merely numbered photographs were dis- 

 tributed as illustrative of the fossils of each period and locality. 

 If such rules were thoroughly heeded by working paleontol- 

 ogists the crop of new species would dwindle, but the species 

 which were entitled to live would be capable of distinct defini- 

 tion and recognition. 



Species and genera can scarcely be called good, so long as 

 the author himself is unable to distribute the typical specimens, 

 twice alike, without reference to the original labels. 



In regard to the genera a word may be said : the author has 

 retained the genus Microdon Conrad, although it is pre-occupied 

 by Agassiz for a genus of fishes, and the genus Cypricardella 

 Hall is acknowledged to be identical, as was shown by Mr. 

 Whitfield (Bull. Am. Mus. Nat. Hist., vol. i, No. 3, p. 63). 

 The first name is used with the second placed parenthetically 



