144 PALEONTOLOGY OF THE EUREKA DISTRICT. 
one-half a millimeter distant from each other, and others where they are 
crowded still more closely; in these we have the typical surface of S. (IL) 
undifera. This crowding together of the concentric striz is carried still 
further in S. (Z.) compacta Meek, where the radiating plications are also 
more numerous. No one can examine the beautiful series of forms given 
by Mx. Davidson on plate vii of his Monograph of Devonian Brachiopoda 
without observing that the range of variation in the strength and number of 
the plications is quite as great as between the Nevada type illustrated on 
plate xiv, and the British American form illustrated by Mr. Meek (Trans. 
Chicago Acad. Sci., vol. i, pl. xiv). The differences in the height of the area 
between the two last-mentioned shells is of a very decided character, but 
among the later collections examples occur that serve to bridge over and 
unite the two in this respect; one specimen has an area of the same height 
as the variety S. (/.) compacta (plate xiv, figs. 13 6, c), and another is inter- 
mediate (plate iii, fig. 5). 
From Lone Mountain there is a shell of the type of S. undifera that has 
about twenty obscure plications on each side of the mesial fold and sinus of 
the ventral and dorsal valve, respectively. These are crossed by concentric 
strice and fine radiating interrupted striz. Still another specimen has lost 
all the radiating plications, and has only the concentric striz and interrupted 
radiating strive (plate iii, figs. 3, 3a, b). The latter shell may readily be 
identified with S. (M.) pramatura Hall, of the Upper Devonian of New 
York, or S. (1L.) pseudolineata or S. (M.) setigera of the Lower Carboniferous 
limestone of the Mississippi Valley. It is, however, in our opinion, a pre- 
cursory type in the Lower Devonian of the shell we have called S. (JZ) 
glaber, var. Nevadensis (ante, p. 139), and which, before the discovery of this 
shell without plications, in the Lower Devonian, was considered the direct 
lineal descendant of S. (1.) undifera. We cannot, however, fail to notice the 
close correspondence between the variety of S. (IL) wndifera described by 
Mr. Meek as S. (J.) compacta, and the Carboniferous S. (JL) pinguis Sow- 
erby, so fully illustrated by Mr. Davidson. S. (MZ.) Richardsoni Meek, from 
the Devonian of British America, is closely allied to the Lone Mountain 
shell having numerous radiating plications. 
From these comparisons it appears that S. (IL) undiferais the type of a 
widely distributed and very variable species. Among the American varie- 
