664 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 127 
treta had very delicate and small hinge plates which contrast quite strongly with 
the large structures of Oligorhynchia. The pedicle beak structures of the two 
genera seem to be identical. 
One poorly preserved brachial interior shows clearly one of the roles of the 
brachial sulcus. This deep exterior trough serves as a median ridge in the interior 
of the valve. The sulcation is so strong and deep that the inner manifestation of 
this structure appears as a high, narrowly rounded ridge. 
Sphenotreta appears first in the Crown Point formation of the Chazy group 
but extends upward into the Sevier formation. It occurs with Oligorhynchia 
in the latter formation. The genus occurs in crystalline limestones or cal- 
carenites and in soft shale. It is in the former type of matrix in the Lenoir and 
Sevier formation but occurs in soft shale in the McLish formation of Oklahoma. 
SPHENOTRETA ACUTIROSTRIS (Hall) 
Plate 124, F, figures 32-38; plate 143, H, figures 37-40 
Atrypa acutirostra Hatt, Pal. New York, vol. 1, p. 21, pl. 4 bis, fig. 6, 1847. 
Rhynchonella acutirostris (HALL), 12th Ann. Rep., New York State Cab. Nat. Hist., p. 65, 
1850. 
Zygospira acutirostris (Hall) Raymonp, Ann. Carnegie Mus., vol. 7, No. 2, p. 227, pl. 34, 
figs. 15-22, IQII. 
Shell small, triangular in outline with the widest part in the anterior 
half; posterolateral margins straight, forming an angle of 80°. Anterolateral 
margins narrowly rounded; anterior margin truncated ; anterior commissure sul- 
cate. Surface marked by Io to 15 narrowly rounded to subangular costae. 
Pedicle valve gently and evenly convex in lateral profile; broadly subcarinate 
in anterior profile. Umbo narrowly convex, marked by a prominent median costa 
which extends from umbo to anterior margin maintaining its prominent char- 
acter throughout. Median costa and 1 or 2 on each side form an indistinct fold. 
Flanks sloping moderately to the margins, not swollen. 
Brachial valve gently convex in the median three-fourths but turned in nar- 
rowly toward the pedicle valve in the anterior quarter. Anterior profile sulcate, 
with narrowly convex flanks and precipitous sides. Umbo and median region 
sulcate; sulcus deep, deepest medially where a narrow groove corresponds to 
the exaggerated median costa of the pedicle valve. Sulcus occupied by 2 to 4 
costae depending on age. 
Measurements in mm.— 
Brachial Greatest 
Length length Midwidth width Thickness 
Hypotype (pedicle valve 118019a)..... 37 ? 2.7 3.4 1.0 
si (brachial valve 118019b).... ? 3.4 Pals 3.2 0.9 
¢ ete HY wEESOOC). or ie 3.6 a, 3.4 0.7 
Types.—Figured hypotypes: Carnegie Mus. 5439, 5440 (one without num- 
ber) ; U.S.N.M. 118015; measured hypotypes: 118019a-c. 
Horizon and locality—Crown Point formation in New York: at Chazy, 
Rouses Point (15’) Quadrangle; Valcour Island, Plattsburg (15’) Quadrangle ; 
Crown Point, Port Henry (15’) Quadrangle. 
