316 PALAEONTOLOGY OF NEW YOKK. 



The casts of the dorsal valve present all the varieties from moderately 

 convex to extremely gibbous. In the younger and less convex forms, there 

 is usually a median depression more or less conspicuous ; the muscular area 

 is not sharply defined, but is distinctly bilobed, the upper parts deeply 

 striated, and a slender ridge in the bottom of the depression. In older 

 shells the muscular area becomes strongly defined and extremely prominent, 

 rising conspicuously above the general gibbosity of the upper parts of the 

 cast. The cast of the rostral cavity is neatly defined and limited on either 

 side by the imprints of the crural bases, while outside of this are the char- 

 acteristic crenulated impressions which mark this part of the hinge-line in 

 well preserved specimens of this and other forms of the genus. 



This species has usually been regarded as a variety of the Atrypa reticularis, 

 which is recognized as so widely distributed in several geological formations. It 

 may be difficult to decide what form shall be assumed as the typical Atrypa reticu- 

 laris, among those which occur in all the formations from the Clinton group to the 

 Chemung group inclusive. When compared with the so-called A. reticularis of 

 the Niagara group, the form is more robust, the ventral valve without sinus, and 

 the dorsal valve flattened or depressed in the middle of the lower half of its 

 length, while the striae are finer and less conspicuous. The same is true when 

 compared with the A. reticularis from the Lower Helderberg group, and also with 

 those of the Corniferous limestone and Hamilton group. • 



Geological formation and localities. This form of Atrypa occurs in the Scho- 

 harie grit, at Schoharie, Knox, Clarksville, Coeymans and other places in New York. 

 It is not known to me in any other geological formation. 



Atrypa reticularis. 



PLATES 51-53 A. 



jlnomia rtticultirU : LiSMJtos, Syst. Kat. ed. xii, p. 1132. 1767. 



Ttrebratula ^ctinata, T. »ubtillU$ime ttriata, T. cancellata, T. minutittime ttriata : ScnROTER, Abhan- 

 dlungeu liber verschicdene Gegenstande der Naturgcschichtc. pi. lii. f 11-18, and pi, iv.,. f. 16-26. 1777. 



TtTtbratula pectinata : BauoniERE, Ilist. Nat. Vers.j Testacds Eucyc. Method, pa. 242, plate iv. 1789. 



jSnomita rtticitlarit: Wahle-ibkro, Nov. Act. Soc. Upssl., Vol. viil, f. 65, 66. 1821. 



Ttrebratula affinit : Sowerbt, Min. Con., tab. 324, f. 2. Jan. 1822. 



Bpiri/er sowerbyi: D» France, Diet., Vol. ix, p. 295, t. Ixxvi, f. 2 (valva brachiis spiralibus ornata). 



Ttrebratula (itfa^tu) cancellata: Eichwald, Zool. i. 276, tab. iv, f. 11. 



Ter$bratulUa priseui : ScnbOTBiiK, Nachtragen zur Petrcfactenkunde, p. xvii, f. 2. 1822. 



