Review of Recent Geological Literature. 249 
had its origin at that time. The position of the eroded sur- 
face of the limestone and the inclination of the directly super- 
imposed strata both seem to indicate that the lateral ravine 
had its beginning at that early date. Of course the principal 
ravine must have existed to furnish an outlet. Along its 
banks the white limestone layer is not exposed, being covered 
with a heavy mantle of drift. In all the quarries worked at 
this point the upper strata show a decided dip towards the 
main ravine, which goes to confirm the theory as to the early 
origin of the drainage system. 
At other localities in this county I have noticed evidence of 
erosion, but in no case was it so pronounced as in the instance 
just given. I expect to find further evidence of disturbance 
ii? the line of unconformity. 
REVIEW OF RECENT GEOLOGICAL 
LITERATURE. 
Tin Lower Silurian FjumeUibranrJiiata <;/' Minnesota. By E, O. Ulrich. 
(( !hapter vi of volume rn of the Final Rept., Geol. and Mat. Hist. Survey 
of Minn., pp. 475-628, plates 35-42; June Hi, 1894.) The structure and 
affinities of the early Palaeozoic lamellibranchs have been but meagerly 
exploited. The stupendous work of Barrande upon these and later 
forms of the Bohemian Palaeozoic has perhaps overwhelmed as much as 
aided students of other Silurian faunas, and as far as pertains to the 
American species .Mr. Ulrich lias had a fair Meld, a magniflcenl oppor- 
tunity, and has acquitted himself admirably and with signal credil to 
the Geological Survey of Minnesota. 
After a few instructive introductory pages on the general relations, 
nomenclature and preservation of these fossils, the author gives a scheme 
of classification for all Palaeozoic genera (in which, however, we observe 
none of those introduced by Freeh and Whidborne). Then follows the 
systematic pari of the work, the principal innovations of which we 
briefly notice. 
The familiar genus Ambonychia, Hall, usually construed from the well 
known form .1. radiata Hall, is here slricth limited to shells having the 
structure of the type, A. bellutriata, that is. without lateral teeth or bys- 
sal opening and with an internal anterior lobe set off i>\ a thin veTtical 
internal plate. 
Clionychia, Ulrich (type, Ambonychia lamellosa Ball), is an elementan 
form without cardinal or lateral teeth or the vertical plate of Ambvny- 
r/l/il. 
