BRACHIOPODA. 167 



retaining only those which conform to the well-known A. reticularis, primarily 

 in the structure of the brachidium, and secondarily, in tlie expression of the 

 exterior. Such forms are comparatively few in number, and most authors 

 have been disposed to regard them as representing unessential variations from 

 the specific type of A. reticularis. There is, however, a multitude of designations 

 which have been applied to contemporaneous variations or consecutive muta- 

 tions of this specific type, some of them unnecessary, but many very useful both 

 to the geologist and the systematist.* 



Atrypa reticularis is a shell characterized by its fine plications, which duplicate 

 rapidly at or between the concentric growth-lines. This duplication or bifur- 

 cation of the plications occurs at irregular intervals in the growth of the shell. 

 It is a secondary condition of growth and if it manifests itself at an early stage, 

 a finer plication results than when its appearance is delayed until later growth. 

 This variability in appearance and rapidity of recurrence produces individual 

 differences of expression in the plication of the shell, which, however, lead to 

 no varietal modifications. 



The concentric growth-lines are bases of free squamae or lamellte, which under 

 favorable conditions may be retained, but are usually abraded, so that the com- 

 mon expression of the exterior is that of an entire absence of sucli growths. 

 This is the condition where the valves have been replaced by silica (a very 

 common mode of retention), or in specimens which have been gathered from 

 compact limestone. Under better preservation, as in soft shales or shaly lime- 



* The time-values of oscillations of, oi- from the specific type, manifest themselves so clearly in this genus, 

 that it is here necessary to express such variations with caution and precision. Barrandk introduced a dis- 

 tinction between primary and secondary moditicalions of a specific type, by propo-sing to restrict the term 

 var'itty to the former, that is, "to forms which possess the principal characters of an admitted species, but 

 which differ from it in one or more important modifications, manifesting themselves in a considerable number 

 of individuals ; " and to designate as variants, secondary modifications of form and surface ornamentation. 

 It would be granted by most investigators that modifications of a specific type more essential than changes 

 in foi-m and surface cliaracters, would be a sufficient basis for a complete separation from such species ; 

 hence this distinction between varuty and '):aria7it, holding the latter suVjordinafe to the former, becomes 

 largely arbitrary ; and it is only in rare instances that any practical use can be made of it. Variant becomes 

 a useful term applied to the different phases of expression within the limit of the specific type, but in this 

 meaning it is neither subordinate to the term variety, nor does it necessarily indicate an incejitive condition 

 in the dex>arture of a variety from the specific type. The time-value of variations from the species has been 

 expressed by Waagen, who proposed to restrict the teim variety to oscillations of the type contemiioraneous 

 with tlie type itself, and applied the term mutation to variations appearing after the extinction of the type. 



