NATURAL SCIENCES OP PHILADELPHIA. 
53 
pressions unknown. Length 2-10 inches ; height 1*10 inches ; transverse 
breadth -70 inch. 
Locality and position. — Western and southwestern base Black Hills, low down 
in bed C of the accompanying section. 
Teredo globosa. 
Shell globose, thin, rounded or subtruncate and gaping posteriorly ; anterior 
hiatus large, and consisting of a rectangular notch extending from the base 
half way up towards the beaks, and back to the middle of the shell. Um- 
bonial region gibbous ; beaks placed near the anterior side, much incurved 
obliquely forward. Surface marked by rather distinct lines of growth, which, 
on the back part of the shell, curve down, parallel to the posterior border, 
until they approach a small indistinct ridge, or slightly elevated line, passing 
down from the umbones, when they curve abruptly upwards nearly parallel 
to the margin of the anterior hiatus, becoming at the same time much finer and 
more regular, as well as very finely and beautifully crenulate ; on reaching a 
small, indistinct groove, which curves down obliquely from the beaks towards 
the corner of the anterior notch, these lines are abruptly deflected upon the 
anterior portion of the shell extending out over the notch. 
The tubes are thin, subcylindrical, and sometimes nearly straight, but gen- 
erally variously curved ; and increase gradually from the smaller to the larger 
end. Length of shell -23 inch ; height and breadth of do. each -22 inch. Di- 
ameter of one of the larger tubes .26 inch. 
Locality and position. — 'Square Butte, near Fort Clark, upper part of No. 5 
of the general section. Found in great numbers in large masses of fossil 
wood. 
Pholas cdneata. 
Shell small, very thin, cuneiform, most gibbous at the anterior end, which is 
truncate, and more or less gaping, — narrowing and much compressed posterior- 
ly ; anal extremity very narrowly rounded and gaping a little. Dorsal margin 
declining slightly from behind the beaks, with a very gentle convex curve to- 
wards the posterior end ; basil margin nearly straight, or a little concave in 
outline. Beaks small, located at the anterior extremity, scarcely rising above 
the hinge, incurved and touching. 
The surface is marked by small concentric wrinkles, which are much more 
distinct on the gibbous anterior half of the shell than behind, and crossed 
by two grooves, the anterior one of which is linear, but well defined (on the 
cast), and extends from the beaks downwards, and a little backwards, so as to 
reach the base in advance of the middle ; the other groove is more shallow, 
broader, less distinctly defined, and extends from the back part of the beaks, 
obliquely backwards and downwards, just within the subangular posterior um- 
bonial slopes. 
There is also a small ridge or elevated line, on the anterior end of each valve, 
curving parallel to the concentric wrinkles, from the base about half way up 
towards the beaks, from which point it is deflected abruptly at right angles for- 
ward, so as to delineate exactly the form of the angular notch or hiatus of 
Teredo and Xyophaga; but the margin of the valve, (at any rate in adult shells) 
extends out beyond this line, so as to leave a comparatively small hiatus. 
The posterior muscular impression is long, very narrow, and placed near the 
postero-dorsal edge ; while the pallial line passes obliquely down near the 
middle of the valves, and appears to be provided with two sinuses, the upper 
one being very small, and lower of medium size. The space behind the pallial 
line is marked on the interior by extremely fine, obscure radiating striae. Length 
•3Y inch; height -18 inch ; breadth -16 inch. 
Iiocality and position. — Long Lake, No. 5 general section. 
1858.] 
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