48 PALEONTOLOGY OP NEW-YORK. 



The interior of the ventral valve is marked by a large fl.abelliform 

 muscular impression, which reaches from one-half to two-thirds the length 

 of the shell. The central or adductor impression is sometimes simple, 

 and sometimes longitudinally divided by a slight median ridge which is 

 stronger below. In the older shells, the ovarian spaces are pustulose. The 

 dental lamellas are strong and divergent, supported below by the ridge 

 which margins the muscular impression. Vascular impressions are rarely 

 seen, extending beyond the muscular area. Under a lens, the interior 

 surface is distinctly punctate. In all well preserved specimens, the exte- 

 rior shows minute tubular openings in the striae ; and when the striae are 

 much worn, these also are seen to be tubular ; while a farther wearing of 

 the surface shows more distinctly the minutely punctate character of the 

 shell. 



This species resembles the figures of 0. michilini of Laveille ; but, compared 

 with European specimens thus labelled, it presents many points of difference. 



The 0. vantcxemi is the most common and abundant species in the Hamilton 

 group. The largest specimens have a length of about one inch and one-eighth, 

 with a width of an inch and a quarter. 



Geological formation and locality. In the calcareous and arenaceous shales of 

 the Hamilton group, in Central and Western New- York. Among the best locali- 

 ties, are the shores of Cayuga, Seneca and Canandaigua lakes ; Moscow ; Darien, 

 and Eighteen-mile creek ; ranging through nearly the entire thickness of the 

 group. In the more arenaceous shales farther east, it is common in the form of 

 casts. It has been recognized in Maryland and Virginia ; and specimens from 

 Iowa have not been proved distinct from this species. 



Ortliis leucosia. 



PLATE VI. 

 OrthU lettcotia : Hall, Thirteenth Report on the State Cabinet, 18G0, p. 80. 



Shell broadly ovate, greatest width below the middle, somewhat obtusely 

 pointed at the beak ; cardinal area short and small, being less than 

 half the width of the shell : beaks approximate. Dorsal valve much 

 the more gibbous, the greatest convexity above the middle, longitudi- 

 nally marked by a median depression which is sometimes obsolete ; 



