STROPHOMENIDZ. 329 
valve with a prolonged and oftentimes bent beak; area trian- 
cular, with a fissure covered by a convex pseudo-deltidium. No 
foramen is observable, but the cardinal process is at times seen 
partially extending under the deltidium. 
Interior of ventral valve, with a strong hinge-work on either 
side at the base of the fissure, supported by a dental plate ; 
muscular sears two, elongated, oval, deeply excavated, separated 
by a wide mesial ridge. 
Interior of dorsal valve with a largely developed cardinal 
process, composed of two projections, grooved or bidentated 
towards the extremity of their outer surface; socket-plates 
large, and partly united to the lower portion of the cardinal 
process; adductor scars quadruple, occupying more than a third 
of the length of the valve, and arranged in pairs, divided by a 
short rounded mesial ridge. 
This genus is intermediate between Orthis and Strophomena. 
MEEKELLA, White and St. John, 1870. Ventral valve without 
septum, with two broad dental lamelle which are continuous 
from the cardinal teeth to the beak, passing directly in front of 
the sutures between the cardinal area and the pseudo-deltidium, 
and thence slightly diverging, they extend forward along the 
bottom of the “valve about fates way to the front, the anterior 
margins of the lamellz arching backward and upward to the 
dentai processes; in the rear of the hinge-line are three cham- 
bers, not communicating together, but all opening into the shell. 
Dorsal valve with long cardinal process, curving backward in 
front of the pseudo- deltidium, which has a wing like expansion 
on each side of it, curved up at its outer edge to form the elon- 
gated dental fosset for the reception of the process of the 
opposite valve. Muscular markings unknown: outer surface 
apparently punctate. Carboniferous; U.S. S. striatocostatus, 
Cox. 
OrtHisina, d’Orb., 1847. 
Syn.—Klitambonites (part), Pronites, Hemipronites, Gonam- 
bonites, Pander, 1830. 
Distr.—Fossil. Lower Silurian; Europe. QO. anomala,Schloth. 
(exxxviii, 41, 42). 
Shell impunctate ? widest at the hinge-line; cardinal notch 
closed, byssal notch (fissure) covered by a convex pseudo- 
deltidium, sometimes perforated by a small round foramen. 
SKENIDIUM, Hall, 1861. (Htym.—Skenidion,a little tent.) Shell 
having the general aspect of Orthis, except in the extreme eleva- 
tion of the ventral valve; cardinal process prolonged into a 
median septum, which extends to the base or front margin of 
the shell, and occasionally bifurcates at this lower extremity. 
Area large and triangular in the typical species. 3 sp. Silur.; 
United States. O. insignis, Hall. 
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