LOWER HELDERBERG ROCKS. 
179 
Orthis varica. 
i 
Plate XXIV. Pig. 1 a - k. 
Delthyns bilobata : Conrad, Ann. Report on the Palaeontology of New-York, 1838, p. 118. 
D. varied : Id. Jour. Acad. Nat. Sciences, Yol. viii, 1842, pa. 262, pi. 14, f. 20. 
Shell subcordiform ventricose, deeply bilobed at the base, with a deep 
sulcus upon each valve reaching to the beaks : hinge line short. Area 
common to both valves; that of the ventral valve higher. Foramen 
high and narrow. Ventral valve arcuate, ventricose on each side of the 
sinus. Dorsal valve gibbous on each side the mesial sinus; each lobe 
becoming, angular towards the beak, and compressed toward the car¬ 
dinal extremities. 
Surface unequally striated; a few distant striae being visible to the naked 
eye, while under a lens the interspaces are seen to be distinctly striated. 
Fine concentric striae cross the radiating striae, and become conspicuous 
towards the base of the shell. 
This species resembles the 0. biloba of the Niagara group; but is a larger and 
more ventricose shell, with the dorsal valve conspicuously more gibbous, and the 
hinge line proportionally shorter. 
Fig. 1 a - g. Figures showing the gradation of size, form, etc 
Fig. 1 k. Enlargement of the radiating strim. 
Geological position and locality. In the shaly limestone of the Lower Helderberg 
group : Helderberg mountains; Schoharie, Catskill, etc. 
This species is rare in all the localities except at the base of the Helderberg near 
Clarksville, where more than 40,000 individuals were collected in the space of half 
a mile square, during a period of ten years. 
