876 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 127 
Interior of pedicle valve with large, stout teeth, short but stout dental plates ; 
muscle field moderately large, subcircular, surrounded by a thin to moderately 
thick extension from the dental plates that nearly surrounds the area. Diductor 
scars crescentic, large; adductor scars small, divided by a short median ridge; 
no pallial marks preserved in specimens available. 
Interior of brachial valve with short, stout cardinal process, convex chilidium, 
and moderately thickened notothyrial platform ; brachiophores thin and bladelike, 
supported by cups of adventitious shell; median ridge short and low, bifurcating 
anteriorly ; adductor scars elongate, separated by an oblique ridge on each side 
of the median ridge. Pallial sinuses consisting of 4 parallel trunks extending 
directly anterior to the adductor field. 
Genotype—Furcitella plicata Cooper, new species. 
Discussion.—This genus is characterized by its fairly strong and even costellae, 
the compressed biconvex form with only the anterior of the pedicle valve flat- 
tened to concave; fairly large foramen; rounded muscle scar in the pedicle valve 
surrounded by extensions from the dental plates ; short dental plates ; strophome- 
noid cardinalia but with median ridge forked at the front and with an oblique 
lateral ridge on each side of it. 
Unlike Strophomena the genus Furcitella appears to have definite dental 
plates. These appear in young specimens in which the margins of the muscle 
area not yet thickened. Close examination of such specimens shows that the den- 
tal plate is directly under the dental ridge and not offset from it. The structures 
have exactly the appearance of the receding dental plate so characteristic of the 
orthids. 
The members of this genus have been previously assigned to Holtedahlina of 
Foerste. The external resemblance between the two genera is very close. Holte- 
dahlina is characterized by a compressed biconvex profile, large foramen, sub- 
equal and strong costellae but the interior is essentially that of Strophomena. 
Inside the brachial valve of Holtedahlina the median ridge does not bifurcate as 
in Furcitella, but 2 ridges arise on each side of it near its anterior end. This 
arrangement simulates bifurcation but is certainly not the same. 
Two other described genera are close to Furcitella: Opikinella Wilson and 
Microtrypa Wilson. The first of these genera is based on two brachial valves 
representing two species O. affinis Wilson and O. salmoni Wilson. This genus 
is characterized by its author as having an unusual septal system. “Three ridges 
lie between the brachiophores. The central ridge divides at a short distance from 
the beak, and the four continue for more than half the distance to the anterior 
margin, the central pair being longer than the lateral pair.” 
The writer has examined casts of these two specimens kindly provided by 
authorities of the Geological Survey of Canada. Examination indicates that the 
median ridge does not divide but that the 2 lateral ridges are implanted on the 
sides of the median ridge near its anterior extremity. This is precisely the con- 
dition that takes place in the septal arrangement of the high Middle and Upper 
Ordovician strophomenas. Accordingly the writer believes that Opikinella is 
a synonym of Strophomena and has so placed it in this monograph. 
