LOWER HELDERBERG ROCKS. 179 



Orthis yaiica. 



Plate XXIV. Fig. 1 a - it. 



Ddthyris bilobata : Coxbad, Ann. Report on the Palaeontology of New- York, 1838, p. 118. 

 D. variea : Id. Jour. Acad. Nat. Sciences, Vol. 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 - ^. Figures showing the gradation of size, form, etc. 

 Fig. 1 k. Enlargement of the radiating striae. 



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. 



