702 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 127 
ends of the didictors. Actually the diductor scars are often separated by a cal- 
losity or elevation at the anterior end of the adductor field. 
The pallial marks of the pedicle valve are deeply impressed in all species of 
the genus. A prominent trunk is given off at the anterior end of each of the 
diductors, one branch of which turns toward the middle and then turns again 
to extend to the front margin. Another branch extends almost directly toward 
the front margin while the third branch turns posterolaterally. All these branches 
send off numerous subsidiary channels. The pallial marks themselves consist of 
narrow channels separated by low ridges so that each main pallial trunk in reality 
consists of two narrow parallel channels. 
The ovarian impressions are usually large and well developed. 
The pseudodeltidium is a narrow arch with its side laid against the deposit 
forming the accessory tooth. The plate is further cemented by adventitious shell 
laid on the sides of the delthyrial cavity and the inner surface of the pseudo- 
deltidium. 
The brachial interior of American Palaeostrophomena is known well in three 
species. These have deep sockets and flat-bladed, rather delicate brachiophores. 
These are distinctly orthoid in structure. The notothyrial platform is moderately 
thickened and is the seat of attachment of a prominent cardinal process. The 
shaft of this structure is short and stout; the myophore is characterized by a 
strong and elevated median ridge which stands between two lower ridges. The 
three ridges are subparallel and separated by deep but narrow grooves. The 
chilidium arches over the myophore and nearly covers it. The brachiophores and 
cardinal process are completely independent. The median ridge is narrow and 
strongly elevated, rising to a point anteriorly and sharply cut off at the front. 
The adductor field is moderately wide at the posterior and tapers to a point an- 
teriorly. The posterior scars are the larger, and both scars may be striated and 
elevated on the margins. 
Pallial marks are prominent in the brachial valve as well as in the pedicle valve. 
As in the other valve the pallial trunks consist of parallel narrow grooves sep- 
arated by low ridges. A major trunk extends anteriorly on each side of the 
median ridge. Another trunk extends more or less obliquely from the point of 
separation of the anterior and posterior adductor pairs. Ovarian impressions are 
strongly developed as in the opposite valve. 
PALAEOSTROPHOMENA ANGULATA Cooper, new species 
Plate 168, B, figures 3-10; plate 194, C, figures 19-22 
Shell small for the genus, wider than long with the width at the hinge varying 
from about 14 to 2 times the length. Cardinal extremities acutely angular, auricu- 
late to submucronate. Lateral margins sloping inward; anterior margin broadly 
to narrowly rounded. Surface beautifully ornamented by costellae of different 
sizes and fine concentric fila. Costellae of three sizes, the larger ones appearing 
in three generations, a median costella stronger and wider than the rest usually 
marks the center of the pedicle valve. Umbonal region usually marked by 3 
