ERACHIOPODA. 



Shell ineqiiivalved, convex or concavo-convex, externally 

 striated ; hinge line rather shorter than the width of the shell ; 

 dorsal valve semicircular, with a small narrow area. Ventral 

 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 (Fig. 26). 



Interior of ventral valve, with a strong hinge-work on either 

 side at the base of the fissure, supported by a dental plate 

 (Fig. 25, t) ; muscular scars two, elongated, oval, deeply exca- 

 vated, separated by a wide mesial ridge (Fig. 26, r). 



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. 



Distribution, 6 species. Sil. — Perm. Europe, Asia, America, 

 and Australia. 



Tropidoleptus, Hall, 1859. 



Etymology, tropis, a keel, and leptos, thin ; the carinated 

 ventral valve and shallow visceral cavity, in its analogy with 

 Leptcena. (See " Reg. Sep.,'' 1856, p. 3.) 



Type, Strophomena carinata, Conrad. 



Shell transversely oval, or longitudinally semi-elliptical, 

 articulating by teeth and sockets, hinge line about equal to the 

 breadth of the shell. Ventral valve convex, with a linear area 

 and triangular foramen in the margin of the area ; from the inner 

 edges of this proceed the dental lamellae, which are separated 

 from the area by a narrow groove strongly crenulated on the 

 outer edge, and extending obliquely outwards, terminating in a 

 low ridge which partially surrounds the muscular impression ; 

 dorsal valve concave ; cardinal process prominent, wedge- 

 shaped, supporting the bases of the crura ; dental fossets crenu- 

 lated, surface plicated ; shell structure punctated. 



Distribution, 2 species. Devonian. United States. 



Vitulina, Ilall, 1861. 



Etymology, Vitula, a goddess. 



Type, V. pustulosa. Devonian. New York. 



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