92 The American Geologist. February, ihqt 
The \vrit(M-'s intention is to i)resent liere an anal3'sis oC Or- 
thh /(^s7/((////r/>vV^ describing the ditferent foi-ins and noting 
tiieii- geologic relations, in ordei' to establish the same as par- 
tial eviden(*e to be used in a Inter discussion on the question 
of relative interval during and between sedimentary divisions 
of the Galena and Maquoketa series. Orlhis festi/dinaria axuit, 
in America is no species at all, but this or that or several 
mixed together. It is O. emacerafd Hall, or O. multisecfa 
Meek, or O. meeki S. A. Miller, or other similar species, which 
are all separated by observable difference? without known in- 
termediate connection. I shall for convenience first describe 
them collectively as 
Orthis festxdina ri(( auct.* 
All have a convex ventral (pedicle) valve with a low me- 
dian fold and an acute curved beak. They have a more or 
less convex dorsal (brachial) valve with a shallow median 
sinus and a beak that scarcely projects. The shell of each 
valve consists of several attached parts (fig. 17, pi. IV), form- 
ing a remarknbly adaptive mechanical structure. The parts 
a re : 
1. An outer thin layer (epidermis) that grows at the mar- 
gin of the sIkMI only. It grows at an equal "rate along the 
whole hinge and hence the flat areas, but from the extremity 
of the hinge to the central anterior margin of the shell the 
rate of growth was increasingly greater. The growth of this 
layer determined the general outline and convexit}" of both 
valves, the folds and sinuses and the plications. 
2. A layer within the first is composed of prisms or "fibres" 
of calcite, that are directed obliquely forward and inward 
.with respect to the whole fossil. They are based against the 
epidermis and end upon the inner surface of the layer. The 
growth was not at the edge of the shell, but upon all the in- 
ner surface outside the area of the muscle scars, (visceral 
area). Its tendency is to completel}^ even up the inequalities 
or plications, so that these in mature shells nearly all disap- 
pear from the inner surface, being, however, apparently repro- 
duced partly anew in senile, very thick shells, in the same, sel- 
dom in a new direction. This layer is punctate, threads of the 
*For o^eneral citations see Davidson'.s Monograph of British Sihirian 
Brachiopoda, p. 226: Winehell and Sebuchert, Geological and Natur.d 
History Survey of Minnesota, final rep., vol. iii, p. 441, 445. 
