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GEOLOGY. 



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'ig. 163. — Ordovician Brachiopods. a, Rafinesquina alternata (Emmons), exterior 

 of the concave brachial or dorsal valve, with the cardinal area and triangular 

 deltidium of the pedicle valve; b, Clitambonites anomala (Schl.), interior of the 

 pedicle valve, showing the spondylum; c-d, Platystrophia lynx (Eich.), exterior 

 of the pedicle valve and anterior view of a complete shell, showing the fold and 

 sinus; e-g, Hebertella sinuata (Hall), interior views of the two valves (e = pedicle, 

 g = brachial) , showing muscular impressions and hinge, and the exterior of a 

 complete shell from the pedicle side; h, Schizotreta ovalis H. and C; the pedicle 

 valve; i-j, Lingulasma schucherti Ulrich, interior of the brachial valve, showing 

 the elevated platform tor muscular attachment, and the median septum; 

 also a lateral view of an internal cast, showing the platforms in each valve; 

 k, Livgula rectilateralis Emmons; I, Trematis millepunctata Hall, pedicle view of 

 a complete shell, showing the unmodified notch-like pedicle opening; m, Schizo- 

 treta ftssus Kutorga, a pedicle valve much enlarged; n, Schizomania filosa (Hall), 

 a brachial valve; o, Crania Iwlia Hall, brachial views of four individuals 

 attached to another shell; p-r, Strophomena subtenta Conrad, (p) profile view 

 of the concavo-convex shell, (q) posterior view of a complete shell, showing the 

 hinge-line, cardinal area, and deltidium, (r) exterior of the concave pedicle valve; 

 s, Orthis tricenaria Conrad, exterior of the brachial valve and the cardinal area 

 of the pedicle valve, showing the open delthyrium; t-u, Rhynchotrema capax 

 Conrad, brachial and anterior views; v. Dalmanella testudinaria (Dal.), brachial 

 view; w, Leptcena rhomdoidalis Wilck.; the pedicle valve ; x, Plectambonites seri- 

 ceus (Sow.), brachial view; y, Catazyga headi (Bill.), brachial view; z, Zygospira 

 recurvirostris (Hall), interior of a brachial valve, showing the spiral brachidium in 

 position; zz, Zygospira exigua (Hall), brachial view. Figures h, i-j, k, I, m, n, 

 and o are inarticulate brachiopods belonging to the orders Atremata and Neotre- 

 mata, and serve to illustrate in a measure the great variety of these brachiopods 

 in the Ordovician fauna. The remaining figures are articulate brachiopods, 

 Protremata and Telotremata. Figures y, z, and zz are spire-bearing Telotremata, 

 which first appear in this period. 



