01© 
PALAEONTOLOGY OF NEW YORK. 
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 Atryjpa 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. 
Anomia reticularis: Linnjetjs, Syst. N&t. ed. xii, p. 1132. 1767. 
Terebratula pectinatce, T. subtillissime striatce, T. cancellatce, T. minutissime striatce: Schroter, Abhan- 
dlungen iiber verschiedene Gegenstande der Naturgeschichte. pi. iii. f. 11—18, and pi, iv., f. 16-26. 1777. 
Terebratula pedinata: Bruguiere, Hist. Nat. Vers.; Testaces Encyc. Method, pa. 242, plate iv. 1789. 
Anomites reticularis : Wahlenperg, Nov. Act, Soc. Upsal., Vol. viii, f. 65, 66. 1821. 
Terebratula affinis : Sowerby, Min. Con., tab. 324, f. 2. Jan. 1822. 
Spirifer sowerbyi : De France, Diet., Vol. ix, p. 295, t. Ixxvi, f. 2 (valva brachiis spiralibus omata). 
Terebratula {Magas) cancellata : Eichwald, Zool. i. 276, tab. iv, f. 11. 
Terebratulites priscus: Schlotheim, Nachtragen zur Petrefactenkunde, p. xvii, f. 2. 1822. 
