402 NATURAL SCIENCE. June. 



Palaeozoic fossils. The first onset at such a group is certain to lay 

 the student low with acute brain-fag, and leave upon his mind a hazy 

 conviction that all the alleged species are pretty much one and the 

 same thing. The beauty of the series and the persistence of its 

 variations or species dawns upon the mind gradually but invincibly. 

 Having attained this point in the study of the original specimens of 

 these fossils, the attempt was made by the process of superposition to 

 ascertain a standard or radicle outline for the entire generic group. In 

 the making of this composite figure, which is here reproduced, it was, 

 of course, necessary to fix upon two points or one dimension as a base 

 to which all the elements could be reduced or elevated : just as in 

 composite portraiture, the eyes, or some other indices, of all faces in 

 the composite must be coincident. In this case the standard to which 

 all have been made to conform has been the distance between the 

 anterior cardinal extremity and the deepest posterior incurvature of 

 the valve ; a purely arbitrary dimension ; probably any other would 

 serve as well. 



" The figure represents superposed outlines of the left valve of 

 36 of these species of Leptodesma. It will be observed that notable 

 differences of outline in the components of this figure are obscured 

 by the coincidence of the lines for a greater, or less portion of their 

 length, while the ruling outline or type stands out conspicuously like 

 a coarsely shaded drawing of the valve. Of all the component out- 

 lines this interwoven valve approaches nearest to that of L. rogersi, 

 as shown in the adjoining figure. It will naturally be understood 

 that L. rogersi enters into this combination no more than the other 

 35 species, nor are there other shells whose outline is so nearly 

 similar to that of L. rogersi as to produce a succession of parallel 

 lines, which thus become intensified. The depth and predominance 

 of the L. rogersi outline is wholly due to the coincidence of parts of 

 outlines of species which vary greatly in this respect, as witness the 

 four silhouettes of elements in this composition, shown on page 403. 



" As already remarked, what is here done for the outline of the 

 valves can be done with equal facility for convexity or other specific 

 variations, either by this method of superposition or by the expression 

 of such differences by a series of co-ordinates or mathematical curves. 



*' One interesting fact in regard to the type or fnndavicntum 

 expressed in this composite is that L. rogersi (the silhouette of 



