814 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 127 
impressions fairly large, bounded on the side nearest the median line by a low 
ridge. Delthyrium open in the adult. 
Brachial valve deeply sulcate, the sulcus on the inside forming a high, rounded 
ridge which serves as a median septum; sockets narrow, slitlike ; brachiophores 
thin, short, flat blades intimately fused with a convex plate that arches over the 
notothyrium ; a septumlike cardinal process and intergrown brachiophores form- 
ing an arched plate suggest the tentlike structure of Sowerbyella; visceral disk 
short, strongly elevated toward the pedicle valve, somewhat expanded laterally 
and produced into a long point anteromedially. 
Genotype.—X enambonites undosus Cooper, new species. 
Discussion.—This genus is one of the most peculiar of all the plectambonitids. 
It will not be confused with any other because of its nearly horizontal wrinkling 
and the elaborate development of the anterior margins. The fold is very strong 
and is also well marked in the young. When adulthood is reached the pedicle- 
anterior wrinkles in the direction of the pedicle valve to form a narrow marginal 
fold which makes 2 concave areas of the anterior portion of the flanks. Internally 
the margin is marked by a carinate ridge which slopes obliquely inward toward 
the middle to form the slip over which the geniculated edge of the brachial 
valve fits. 
The interior of the pedicle valve is somewhat suggestive of Leptellina in its 
short and wide, subrectangular muscle area which is lodged in the delthyrial 
cavity. The lateral margin is formed by vestigial dental plates which are barely 
visible. The anterior end of the muscle area is excavated to form an inverted 
V-shaped chamber as the median part of the valve deepens. A second V-shaped 
chamber may be developed in the trough of the pedicle valve anterior to the 
muscle area. This one is probably created by the main trunks of the vascula 
media which travel anteriorly on each side of the median trough. Not one pedicle 
valve showed clear-cut trace of a pseudodeltidium. The genus probably had one 
in young stages, but none of the young specimens preserves it. 
The brachial valve has the anterior margin strongly geniculated toward the 
pedicle valve. This geniculated edge fits over the carinate subperipheral rim in 
the pedicle valve. The brachial valve thus forms a boxlike lid to the pedicle valve. 
None of the brachial valves showed any clear trace of a vertical median sep- 
tum on the tentlike brachial structures. The preservation is not good, but if 
such a septum were present a trace of it should appear. The arched plate which 
is welded to the brachiophores overhangs the peculiar visceral disk. The latter 
is short but strongly elevated. It is narrowly folded in the middle to form a low 
median ridge which extends somewhat obliquely toward the pedicle valve and 
is produced at its anterior extremity into a long, sharp point. The sides of the 
disk cover the anterior parts of the valve. No trace of pallial marks was seen on 
this portion of the valve, and the muscle marks were not differentiated in the 
visceral disk. 
No genus is at present known to which this one will closely compare. 
