FOSSILS OF THE SILURIAN AND DEVONIAN ROCKS. 93 



the dorsal valve is also deflected upward at the base and in the basal quarter. 

 Surface covered by fine thread-like striae, which increase towards the margins 

 by bifurcation ; there are, also, tine concentric lines of growth. 



Formation and Locality. Found abundantly and as well preserved specimens in the Niagara rocks 

 in Jefferson county, Ky., and in Clark county, Indiana. It differs from its Devonian cousin by its consid- 

 erably smaller size, which seldom exceeds that of the specimens illustrated on plate 32, and also by the 

 finer and elegant ornamentation u[ its suri'uco, 



Genus Cyrtia. Daiman. 



Cyrtia, Daiman. Kongl. Vet. Acad. Handl. 1827. 

 Etymology : Kyrtia, a fishing basket. 



This genus includes certain spiriferoid forms, possessing semi- conical or 

 semi-pyramidal ventral valves, high, flat areas, with a narrow fissure closed by 

 a convex pseudo-deltidium. Some of the species have a round perforation in 

 the upper part of the pseudo-deltidium ; but this feature is not constant; it 

 may be present or absent, and has not been considered of generic significance. 

 The types of Daiman' s genus, Cyrtia, are C. exporrecta and C. trapezoidalis, 

 which are now considered identical. In 1858, Mr. Davidson separated several 

 species, which were then classed with Cyrtia, from it, because they did not 

 correspond with the types in several important points, and placed these in the 

 genus Cyrtina. Both genera differ in the shell structure, which in Cyrtia is 

 impunctate, while it is punctate in Cyrtina. The dental plates of the ventral 

 valve in Cyrtia are similar to those of Spirifera, while those in the ventral 

 valve of Cyrtiua are greatly different and show a peculiar modification. 



Cyrtia exporrecta. WAHLKNBBRQ. 



Plate XXVII., figures 6, 7, 8 and 20. 



Cyrtia exporrecta, Wahlenberg. Nova. Acta. Regiae. Soc. Sci., Vol. 8, Niagara group 1821. 

 Spirifera (Cyrtia) trapezoides. Hall. 24th Rep. N. Y. State Museum, p. 1831872. 

 Oyrtia trapezoides, Hall. 27th Rep. N. Y. State Museum, pi. 9, figures 19, 20 and 211875. 



Shell below medium size, pyramidal when resting on the dorsal valve ; hinge 

 line longer than the greatest width of the shell, but sometimes a little less. 

 Cardinal extremities mostly acute, sometimes obtuse, but never rounded. Sur- 

 face smooth without plications. Ventral valve regularly pyramidal, forming 

 at the apex an almost right-angle triangle. Mesial sinus extending from apex 

 to front, where it forms a medium sized quadrilateral extension, well defined, 

 but shallow and rounded or flattened in the bottom, and rather narrow in 

 front. Cardinal area greatly elevated, sharply defined in its margins, more or 

 less concave, being mostly vertical, but sometimes leaning a little either to the 



