BRACHIOPODA. 167 



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

 in the structure of the brachidium, and secondarily, in the 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 lamellae, 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 such 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 This is the condition where the valves have been replaced by silica (a very 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-valnes of oscillations of, or fi-om the specific type, manifest themselves so clearly in this genus, 

 that it is hei-e necessary to express such vainations with caution and pi-ecision. Bahrande introduced a dis- 

 tinction between pi-imary and secondary modifications of a specific type, by proposing- to restrict the term 

 ixtritty lo the former, thjit is, "to forms which possess the principal charactei-s of an admitted specie?, but 

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

 of individual.^ ; " ami to designate as variants, secondary modifications of form and surface ornamentation. 

 It would Ijegi-anted by most investi^ratoi-s that modifications of a specific type more essential than changes 

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

 hence this distinction between varUiy and varia7it, holding the latter subordinate to the former, becomes 

 largely arVntrary ; 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 necessai-ily indicate an inceptive condition 

 in the departui-e of a variety from the specific type. The time- value of variations from the species has been 

 ex|>re88ed by Waaobk, who proposed to resti-ict the teim variety to oscillations of the type contemjioraneous 

 with the type itself, and applied the tei-m mutation to variations appearing after the extinction of the type. 



