980 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 127 
as low ridges bounding the muscle area. Pedicle attached to a smail apical plate 
fixed on the sides of the dental plates at the apex. Muscle area elongate, with 
elongate and narrow diductor scars. Adductor track narrow, not elevated. 
Brachial valve with a deep notothyrial cavity formed by the convergence of 
the brachiophore plates to unite with the median septum. Brachiophores long 
and broad, grooved on the outside and forming an inverted S in section. Median 
ridge strong and high. Cardinal process a low and simple ridge on the floor of 
the notothyrial cavity. 
Genotype.—Laticrura pionodema Cooper, new species. 
Discussion—tThis genus is best characterized by a combination of exterior 
and interior features. The known species all have an elongate beak and a long 
interarea with long, narrow delthyrium. The surface is marked by costellae, usu- 
ally not brought into fasciculae. Isolated swollen and hollow costellae as in 
Pionodema are often present, and one species was individualized on the strength 
of their development. 
Inside the pedicle valve the strong, receding dental plates are distinctive. The 
anterior ends of the plates are usually continued anteriorly to the front end of 
the muscle scars. The teeth are small but have deep fossettes. In some speci- 
mens, particularly of L. latibrachiata, a small plate appears in the apex of the 
delthyrium. This plate is short and is located on the sides of the dental plates 
between the floor of the delthyrial cavity and the surface of the delthyrium. 
The most distinctive feature of the brachial valve is the large brachiophore 
with its great length and width. This is anchored to the median ridge by con- 
verging brachiophore supporting plates. These form a narrow notothyrial cavity 
not unlike that of Scaphorthis and Corineorthis among the impunctate genera 
and Linoporeila of the punctate genera. The cardinal process is a thin, simple 
blade on the floor of the notothyrial cavity. No other punctate genus is similar 
to this one in interior details, the brachiophores being unique. Linoporella is 
similar in the form of the notothyrial cavity but is externally different and is 
not provided with the enormous brachiophores. 
LATICRURA HETEROPLEURA Cooper, new species 
Plate 143, F, figures 25-29; plate 221, A, figures 1-6 
Large for the genus, wider than long, subrectangular in outline; hinge not 
quite equal to widest part which is at about the middle; sides gently rounded ; 
anterolateral extremities narrowly rounded; anterior margin broadly rounded; 
anterior commissure sulcate ; surface costellate, costellae numbering about 4 or 5 
to the millimeter at the front margin; surface marked also by numerous swollen 
and enlarged costellae. 
Pedicle valve with elongate and erect beak; umbo swollen, narrow; lateral 
profile gently convex; anterior profile narrowly convex in the median region but 
with long and moderately steep lateral slopes; middle narrowly convex and 
forming a poorly defined fold. Flanks slightly convex. 
Brachial valve unevenly convex in lateral profile, the posterior half gently con- 
