782 Report oF THE STATE GEOLOGIST. 
trilobed, the lateral lobes bearing the crural bases. The crura 
are straight and their attachment to the primary lamelle is of 
the same character as in Criornyris, etc. The primary lamelle, 
on the umbonal curve, are broad, the jugum usually situated 
posteriorly. The saddle of the jugum is often bilobed on its 
anterior margin, and frequently both it and the outer margins of 
the ribbon of the secondary volutions are fimbriated. 
The muscular impressions of this valve are very narrow, and 
subdivided into two pairs of elongate scars. The members of the . 
posterior pair are divided by a median septum or ridge, which 
begins beneath, though it does not support the hinge-plate- 
Branching vascular sinuses are sometimes retained over the 
pallial region of both valves. 
Surface of the valves smooth, that is, with sharp, concentric 
strie which were never produced into lamelle. _ 
(Type, Seminula ambigua, Sowerby (sp.). Carboniferous.) 
Subgenus Spirigerella, Waagen. 1883. 
Shells elongate transverse; contour showing a decided ten- 
dency to plano-convexity, the pedicle-valve being depressed by a 
broad, flat sinus, and the brachial valve considerably elevated ; 
cardinal slopes more or less pronounced ; 
surface smooth or with sharp, concentric 
growth-lines, which were not produced into 
lamellz or spines. On the interior the hinge- 
plate is high, the anterior face being erect, 
Fic, 315.—Jugum of Spi- 
rigerella Derbyi, Waagen. the upper face subquadrate in outline and 
CAREEM) concave, the posterior face extending con- 
siderably beyond the hinge ; it is perforated by a visceral fora- 
men. The jugum is situated pretty well back and its structure is 
essentially like that in Actinoconchus planosulcatus ; in S. Derby2, 
however, the saddle of the jugum, which is entire on its anterior 
margin, bears a median septum on its summit, extending 
from its anterior edge to the bifurcation of the stem; a 
feature not elsewhere observed among the athyroids, except 
in KaysErya. 
(Type, Spirigerella Derbyi, Waagen. (7?) Carboniferous.) 
34 
