BRACHIOPODA. 203 



culiar. The shell is transversely oval, with a straight hinge-line and rounded 

 cardinal extremities ; its form is depressed plano-convex, the convexity of the 

 pedicle-valve being slight. The cardinal area is about equally developed on 

 each valve. The surface is ornamented by fine, rounded, radiating strias be- 

 tween each two of which are from two to six much finer radiating lines ; all 

 these are crenulated by exceedingly faint growth-lines. 



The interior of the pedicle -valve has short teeth with prominent extremi- 

 ties, and inconspicuous dental lamella. The muscular impression is large and 

 consists of a small adductor scar situated centrally, and two lateral scars which 

 are flabellate and greatly elongated, but not uniting in front and enclosing the 

 adductor as is usual in typical forms of Orthis. These lateral scars are divided 

 into subordinate impressions, of which the outer posterior members may rep- 

 resent the adjustors. In the brachial valve the dental sockets are obscure, the 

 crural plates oblique, terminating abruptly at the bases of the crura, their 

 lower part being continued in a low ridge surrounding a short subcircular 

 muscular area, which is very obscurely quadrilobate. The cardinal process 

 has a vertical, sharp, simple, posterior edge, but is much thickened where it 

 unites Avith the crural plates, and is produced along the muscular area as a 

 prominent median ridge. From the ante-lateral margins of the muscular area 

 radiate six low, somewhat sinuous ridges of similar character to those frequently 

 seen in members of Group XI. Both valves are considerably thickened just 

 within the margins, as in many strophomenoids. 



The shell-structure is finely fibrous and perforated by minute punctations 

 which are in general sparsely developed, but most distinctly arranged in 

 radiating rows corresponding to the surface stri^. 



Orthis Clytie is the only representative of this type of structure known in the 

 American Silurian, but with it may be associated Sowerby's 0. alternata* and 

 McCoy's f 0. retrorsistria, both from the Caradoc horizon. The former of these 

 is a very close ally to the American Trenton form in all its external specific 

 characters. 



*So\VERBY, in Murchison's Siluvian System, p. 63S, pi. xix, fig. 6 ; 1S39, and Davidson, Silurian Braohio- 

 poda, p. 264, pi. xxxi, figs. 1, 3, 7 ; Suppl. p. 187, pi. xiv, figs. 1-6. 



t McCoy, British Palaeozoic Fossils, p. 224, pi. i h, figs. 12, 13 ; 1852, and Davidson, Silui-ian Brachiopoda, 

 p. 265, pi. xxxi, figs. 2, 4-6 ; pi. xxxvi, figs, 39-42 ; Suppl. p. 185, pi. xiv, figs. 7-16. 



