848 Report oF THE STATE GEOLOGIST. 
Muscular impressions not clearly defined in the pedicle-valve, 
but in the brachial valve are two oblong or subovate scars a little 
below the beak, one on each side of the median line 
Type, Stricklandinia Gaspensis, Billings. 
Distribution. Upper Silurian. 
Amphigenia, Hall. 1867. 
(Plate 49, figs. 8-15.) 
Shells elongate-ovoid, high-shouldered, broadest behind and 
sloping to a narrow curve anteriorly ; without median fold and 
sinus. Surface smooth, with irregularly distant concentric 
growth-rings and faint radial striz. There is no cardinal area, 
and deltidial plates are not developed in the large triangular 
delthyrium. In the pedicle-valve is a short spondylium formed 
by the union of the dental lamelle with the median septum. 
The muscles were implanted upon this spondylium and there is 
no evidence of scars elsewhere upon the valve except those left 
by the vascular sinuses upon the post-lateral surfaces. In the 
brachial valve is a large subquadrate hinge-plate, perforated by 
the visceral foramen which with age becomes filled up by 
adventitious deposits. The crura are long, straight, inclining 
upward or toward the opposite valve and are expanded at their 
extremities into palmate processes. The muscular scars in this 
valve are sharply developed, forming together an elongate 
adductor area, clearly divisible into an attenuate anterior pair 
and a broader posterior pair. The former have about twice the 
length of the latter and are marked by transverse, fine, closely- 
set wrinkles; the latter are radiately and coarsely striate. From 
the posterior termination of this area to beneath the hinge-plate 
extends a broad, smooth sinus, from which is given off a pair of 
strong lateral branches, which ramify over the genital area in 
the umbonal region. 
The substance of the shell is impunctate on the surface- 
Beneath the epidermal layer is another that is highly punctated ; 
within this, and apparently forming the innermost lamina, is a 
third layer whose surface is minutely wrinkled. 
Type, Amphigenia elongata, Vanuxem (sp.). 
Distribution. Lower Devonian (Corniferous limestone), 
100 
ee 
