612 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 127 
slender, nearly straight ; fulcral plate long and slender forming a deep trough on 
the outside of the brachiophore; posterolateral face of brachiophore thickened 
and overhanging socket, terminating in a fairly long triangular accessory tooth. 
Brachiophore plates long, high, slender, confined to the posterior quarter or less ; 
muscle scars not seen. 
Genotype.—Parallelelasma pentagonum Cooper, new species. 
Discussion.—This genus shows many peculiar characters, both internal and 
external. The inner anatomy suggests relationships to Salonia, Metacamarella, 
and to Pentamerus itself. It is a remarkably complicated pentamerid for one of 
the earliest known. 
At first glance the exterior of this genus suggests a camerellid. It is much 
narrower and more attenuated at the beaks than is normal in that genus. It also 
flares out anteriorly and has straighter posterolateral sides than is usual in 
Camerella. The sides of both valves of Parallelelasma are inflected and flattened 
or concave. In side view the genus, in its posterior half or two-thirds, would 
have a very compressed appearance. The lateral commissure is often broadly 
serrate to the point where the pedicle tongue appears. Here the right-angled in- 
flection is terminated. At the place where the flattened lateral area terminates is 
a narrow gap in the pedicle valve to receive the lobe marking the anterior termi- 
nation of the lateral inflection of the brachial valve. The pedicle tongue is short 
and broadly rounded. The anterior commissure of the brachial valve is either 
rectimarginate or with a broad wave to fit the broadly rounded and serrate pedicle 
tongue. The material available does not show a marked anterior broad fold as 
is usual in many pentameroids. 
The exterior is paucicostate. The posterior half or less is smooth, but the 
anterior half or more is marked by broad, low, rounded costae. These present 
no unusual features as they appear on the body of the shell, but their terminations 
along the anterior margins of both valves are unusual. The fairly strong costae 
terminate in deep reentrants, while the grooves between the costae end in long, 
sharp projections to produce a jagged margin. The reentrants are of particular 
interest. 
The costated anterior edge is modified to form an elaborate type of strainer. 
The edge of the reentrant is deflected anteriorly to form a low marginal wall. 
Across the reentrants of each valve a sort of shelly diaphragm is built slightly 
obliquely from the shell surface. These diaphragms across the reentrant com- 
bined with the low wall built around the entrant define small chambers at the 
ends of the costae. The costae are alternate in arrangement which permits the 
acute projection extending from the troughs to fit into the chamber at the end 
of the reentrant. The end of the trough fits over the diaphragm closing the re- 
entrant. This neat device forms a very efficient straining device for the food- 
bearing currents entering the valve. 
The interior of the pedicle valve shows no unusual features. The spondylium 
is short, fairly shallow and has a long anterior projection at its free end. The 
articulating process or tooth is of interest because it is so long and slender. 
Another feature of interest in connection with the articulation of the pedicle valve 
