810 PALEONTOLOGY OF NEW- YORK. 



Rliynelionella Ycnustnla. 



PLATE LV. 



Jitiypa cuhoidtt : Vahuxkx (non Sowebbt), Geol. Report Third Dist. New-York, p. 163, f. 1. 



•• M ! Hall, Gool. Report Fourth Dist. New-York, p. 215 and 216, f. 1. 



Compare Atrypa cuboidet .- Sowerbt, Geol. Transactions, 2d series. Vol. v, pi. Ivi, f. 23. 



Also <• '• FaiLin, Palssozoic Fossils of Cornwall and Devon, pa. 85, pi. ^exxIt 



f. 160. 

 Also Tertbratula crtnulata : Id. ib. p. 85, f. 152. 



A\ao Atrypa tubcuboidet, D'Oebiony = jl. cuboidet, Philips (non Sowerdt), Prodrome, Vol. i, 



p. 93, n. 884. 



Shell in the young state oval or ovate, with, a moderate or scarcely per- 

 ceptible sinus, and few or no plications except upon the middle of the 

 shell ; becoming gibbous, rotund or subcuboid, with the width a little 

 greater than the length, and the depth from two-thirds to three-fourths 

 as great as the length, with a deep sinus and the surface entirely pli- 

 cated ; cardinal line extending for two-thirds or three-fourths the 

 width of the shell. 



Ventral valve in young shells a little less convex than the dorsal, and 

 moderately or scarcely at all depressed in the sinus. In older shells 

 the ventral valve is moderately convex in the middle above, and 

 abruptly depressed in the broad deep sinus, while it is flat or concave 

 towards the lateral margins. Umbo moderately elevated, with the 

 beak minute and closely appressed. The cardinal margins are often so 

 abruptly incurved as to give the appearance of a narrow area on each 

 side of the apex. 



Dorsal valve in young shells a little more gibbous than the opposite, 

 and without defined mesial elevation ; becoming more gibbous with 

 the growth of the shell. In full-grown specimens the dorsal valve is 

 very gibbous, with a defined mesial fold reaching above the middle 

 of the shell, sharply truncate in front, and usually bearing six or seven 

 plicaations; the sides of the shell curve very abruptly and almost ver- 

 tically to the opposite valve. The apex is very abruptly incurved 

 beneath the beak of the opposite valve. 



Surface in young shells marked by about four plications in the mesial 

 sinus ; while there are from ten to fifteen on the sides of the shell, 



