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TRENTON LIMESTONE. 121 



161. 8. ORTHIS FISSICOSTA ( 71. «;>.). 



Pl. XXXII. Figs. 7 a, b. 



Compare Orthis ealligramma. Dalman, Hisinger, de Verneuil. 



Orthia actonue. Sowehby in Mithchison's Sil. System, pag. 639, pl. 20, fig. 16. 



SemioTal, with the cardinal line somewhat less than the width of the shell ; area narrow ; 

 foramen narrow, triangular, reaching to the apex ; dorsal valve moderately convex, with 

 the beak extending and slightly incurved ; ventral valve moderately convex in the middle, 

 and depressed at the sides ; surface marked by angular costae, which become bifid and trifid 

 on the centre or towards the margin of the shell ; number of costae about 19 or 20, 



This shell, in its general form, resembles the 0. ealligramma ; but it is clearly a distinct 

 species, judging from a single European specimen in my possession. The bifid, trifid, and 

 even quadrifid costae are an obvious difference, though such a change may take place in 

 the costae of 0. ealligramma. The costae are fewer on this species, and more elevated ; the 

 ventral valve is convex, while in that species it is flat and slightly depressed in the centre ; 

 the dorsal valve is less convex in our species, though the specimen is somewhat crushed. 



In the divided character of the costae, this shell resembles the Orthis aeionite cited above ; 

 but our specimen is smaller than the one figured by Sowerbv, and has a greater number 

 of cost£ (19 on the dorsal and 20 on the ventral valve, while that one has 14). A com- 

 parison with a single specimen of O. actonia shows the costae to be more distant in that 

 species than in ours. 



Fig. 7 a. View of the ventral valve, showing a portion of the area and beak of the dorsal valve. 

 Fig. 7 b. Profile view of the same, showing the projecting beak of the dorsal valve. The dorsal valve 

 has been crushed, so that the profile view does not give a correct idea of its convexity. 



Position and locality. This species occurs with the preceding ones in the Blue limestone 

 of the West, but it is unknown to me in New- York. I obtained but a single specimen near 

 Cincinnati, showing that the shell is comparatively rare. 



162. 9. ORTHIS TRICENARIA. 



Pl. XXXII. Figs. 8 a, b, e, d, e. 

 OrthU trictnaria. CoKmAO, Proc. Acad. Nat. Sciences, 184-3, Vol. i. p. 333. 

 Semioval ; length and breadth nearly equal ; cardinal line straight, equal to, or greater 

 than the width of the shell ; area very broad ; foramen long, sublineor ; dorsal valve sub- 

 conical, with the apex much elevated and produced, though scarcely incurved ; cardinal 

 margins sloping very obliquely from the beak ; ventral valve flat, regularly rounded from 

 the extremities of the hinge line ; surface marked with from 16 to 32 rounded simple 

 radii, with spaces between equal to the radii. 



The most striking character of this shell, and one which will serve to distinguish it from 

 any other Orthis known to me in New- York, is the great elevation of the dorsal valve, and 

 remarkably broad cardinal area ; the ribs are always simple, continuing distinct quite to 

 the apex of each valve, and gradually enlarging towards the base ; the foramen is some- 

 what linear in perfect specimens, and continues quite to the beak. 

 I Palaeontology.] 16 



