ATHYRIS OF THE CHEMUNG GROUP, 293 



The muscular imprints of the dorsal valve are not well preserved in 

 any specimens in my collection. There is a slender longitudinal septum 

 extending from the beak for one-half the length of the valve. 



This species is distinguished from the otiiers here described by the great ine- 

 quality of the valves, aud by the lamellose strife which are not unlike those of A. 

 vittata in the Hamilton group of the west. The specimens are for the most part 

 poorly preserved, and occur as single valves ^thich are usually partially exfoliated. 

 Casts of the interior arc not unfrequent, but the material is usually too coarse to 

 preserve the finer markings. 



The figures 9-13 of Plate xlvii illustrate the general characters presented by this spe- 

 cies from several localities in Allegany and Cattaraugus counties. Figure 14 is the cast of 

 a small ventral valve. 



Figure 15 is the cast of an elongate form of the ventral valve. Figures 16-19 are illus- 

 trations of the cast of a gibbous form from the arenaceous beds in Cattaraugus county. 



There may, perhaps, be some doubt whether this species will prove a true Athyris. 



Geological formation and localities. This species is known to me only in the 

 Chemung group. It occurs in the Genesee valley at Philipsburgh and Eockville, 

 Allegany county ; near Grcat-vallcy, Randolph and Cadiz in Cattaraugus county j 

 and at Mcadvillc in Pennsylvania. 



Atliyi'is ? jiolita. 



PLATE XLVII. 

 Atrypa polila : Hah., Report on Fourth Geological Dist. N. T. Tables of Organic Remains, No. 66, 

 fig. 5. 1843. 



Shell subglobose or short ovoid, more gibbous in the upper part, with a 

 distinct mesial fold and sinus. 



Ventral valve scarcely more gibbous than the dorsal, the greatest gib- 

 bosity above the middle, flattened near the beak and at about one-third 

 the length becoming depressed in an undefined sinus which expands 

 and becomes deeper towards the front ; beak closely incurved over 

 the umbo of the opposite valve and perforated at the apex. 



Dorsal valve with its greatest gibbosity a little above the middle, and 

 flattened in the centre, curving abruptly to the beak and lateral mar- 

 gins ; a broad mesial fold becoming defined at about one-third the 

 length from the apex, flattened /above, and very little elevated at the 

 sides. 



