PLA S TICI TV A ND PER MA NENC Y OF CHA RA C TERS. 3 I / 



Linnean form ' ' varies like all those species w/iieh possess an eX' 

 tended Iiori.'^ontal a Jid vertical distribution." Barrandc recoof- 

 nized two varieties of the species var. Verncuiliana and var. 

 MurcJiisoniana. McCoy in " British Silurian Fossils," says 

 *' It varies, firstly, in the convexity of the valves, both as to 

 degree, distance from the beak (at which it is greatest), and 

 equality — some small varieties, and the young at all times, 

 having the valves almost equally and evenly convex ; secondly 

 in form, some, and particularly the young and small varieties, 

 being nearly orbicular; others being elongate, and nearly 

 triangular from the width of the hinge-line and narrowness of 

 the front ; thirdly, in the number, thickness, and closeness of 

 the ridges and the scales which cross them, both of which are 

 often smaller and closer than in the typical variety ;" and Lind- 

 strom, speaking of the coarse-ribbed specimens in Gothland, 

 says, "these variations are connected with the finely-ribbed 

 varieties by every possible gradation and intermediate shape." 



These opinions were written by naturalists looking upon 

 species from the old point of view of immutability, but it will 

 be noticed that the testimony is unmistakable as to the great 

 range of i)icessant variation exhibited by the species. 



Hall's Comment on the Variability of the Species. — James 

 Hall, the veteran American paleontologist, in one of his latest 

 and ripest publications,* speaking of the genus Atrypa, says: 

 " Following closely the foregoing diagnosis will result in elim- 

 inating from this group the great majority of species passing 

 under the name of Atrypa, and in 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 desig- 

 nations which have been applied to contemporaneous varia- 

 tions or consecutive mutations of this specific type, some of 

 them unnecessary, but many very useful both to the geologist 

 and the systematist " (pp. 166-7). 



* " Introduction to the Study of the Genera of the Paleozoic Brachiopoda " 

 (1893). 



