STROPHOMENID^. 329 



valve with a prolonged and oftentimes bent beak ; area trian- 

 gular, 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 scars two, elongated, oval, deeply excavated, sepai'ated 

 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 lamellee 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 half-way to the front, the anterior 

 margins of the lamellte arching backvfard and upward to the 

 dental 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. striatocosfatus, 

 Cox. 



Orthisina, d'Orb., 1847. 



Syn. — Klitambonites (part), Pronites, Hemipronites, Gonam- 

 bonites. Pander, 1830. 



Distr. — Fossil. Lower Silurian ; Europe. 0. ano7naZa,Schloth. 

 (cxxxviii, 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. {Etym. — 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 tliis lower extremity. 

 Area large and triangular in the typical species. 3 sp. Silur. ; 

 United States, 0. insigyiis, Hall. 

 22 



