144 PALEONTOLOGY OF THE EUKEKA DISTK1CT. 



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 8. (M.) 

 undifera. This crowding together of the concentric striae is carried still 

 further in 8. (M.) compacta Meek, where the radiating plications are also 

 more numerous. No one can examine the beautiful series of forms given 

 by Mr. 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, pi. 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. (M.~) compacta (plate xiv, figs. 13 ft, 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 

 striae and fine radiating interrupted strise. Still another specimen has lost 

 all the radiating plications, and has only the concentric striae and interrupted 

 radiating strise (plate iii, figs. 3, 3 a, 6). The latter shell may readily be 

 identified with S. (M.) prcematura Hall, of the Upper Devonian of New 

 York, or S. (M.) 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. (M.) 

 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.(M.} undifera. We cannot, however, fail to notice the 

 close correspondence between the variety of S. (M.) undifera described by 

 Mr. Meek as S. (M.) compacta, and the Carboniferous S. (M.} pinguis Sow- 

 erby, so fully illustrated by Mr. Davidson. 8. (M.) Eicliardsoni 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. (M.) undifera is the type of a 

 widely distributed and very variable species. Among the American varie- 



