Geslogy and Paleontology. 1015 
But that conditions eminently favorable to the existence of 
life finally took place, is demonstrated by the fact that from a 
few feet above the base of the formation to the top of it, the strata 
irige to repletion with various normal, and often large, forms 
of life. 
As will be observed by the enumeration, Brachiopods form 
but an inconspicuous feature of the fauna at the base of the shales ; 
while the fauna of the higher horizons of this formation shows a 
preponderance, both as to numbers of species and individuals, of 
this class of organic remains. 
In general, the different classes of fossil remains, as Brachiopoda, 
Gastropoda, Crustaceans, etc., are restricted in their vertical range 
to certain definite horizons, and so do not occur in equal force 
throughout the entire formation. 
This paper, together with others published and in press, are 
Shale, to a Monograph on the Devonian formation of 
owa. 
Rhynchonella subacuminata, n. sp.—Shell somewhat variable; 
sub-triangular in marginal outline; greatest width above the centre 
of the shell; contracting quite rapidly to the front, where it termi- 
nates in three sharp angles, which are produced by the sharply 
angular folds on the front of the valves. Dorsal valve strongly 
convex in the centre; furnished with three prominent sharply 
angular folds at the front, which usually become obsolete before 
reaching the centre of the shell; sinus, large, deep, and broadly 
rounded; margined in front by from two to three sharp, short 
folds ; front and cardinal margins sharply serrate. Surface of shell 
smooth ; texture fibrous. In the young specimens of this species, 
folds or elevations are not present on any portion of the shell. 
Position and locality : Rockford Shales, Hackberry, Cerro Gordo 
county, Iowa. 
Athyris minutissima, n. sp.—Shell minute; subovate in outline ; 
valves strongly and nearly evenly convex ; greatest convexity above 
the middle; cardinal margins sloping rapidly to slightly below the 
Centre, thence rather broadly curving to the front. Ventral 
valve slightly more convex than the dorsal valve; umbo 
prominent ; beak sharp, and incurved over the beak of the oppo- 
site valve. Dorsal valve slightly less convex than the opposite 
valve; umbonal region strongly convex. Surface marked by strong 
equi-distant imbricating lines. Neither valve has an elevation or 
depression at the front that would correspond to a mesial fold or 
sinus, 
Dimensions: length, 23 mm.; greatest width, 2 mm. 
Position and locality: lower portion of Rockford Shales, Rock- 
ford, Floyd county, Iowa. 
