448 PALEONTOLOGY OF NEW-YORK. 



The hinge-line is often much extended, and in the dorsal valve nearly 

 straight to the cardinal angles. There is sometimes the appearance of a 

 false area on the ventral valve, somewhat similar to Atrypa, the margin 

 being thickened and grooved ; but this does not appear to be a characte- 

 ristic or constant feature. 



The specimens of the interior, which have fallen under my observation, usually 

 preserve only the short crural processes; and it is in one specimen alone, which is 

 partially filled with crystalline matter, that distinct cavities can be seen correspond- 

 ing with what I have described. The crystalline matter was first deposited ujx)n 

 these internal organs, which have subsequently almost entirely decomposed, leaving 

 in tlie cavities fragments of the substance, showing the original form of the crura 

 and appendages. 



The shells of this form have been described as Terebratula, and more recently 

 have been included with Rhynchonella, from which they differ conspicuously in 

 the great inequality of the valves and the extension of the hinge-line, as well as in 

 their internal structure. 



I have heretofore referred to this genus the Leptocalia {Jltrypa) disparilis of the 

 Niagara group, the Terebratula lepida of Goldfuss, the T. sublepida and T. duboisi of 

 MM. MuBCHisoN, DE Verneuil and Keyserling ( Geology of Russia and the Ural 

 Mountains). The L. concava and L. imbricata of the Lower Helderberg group are 

 analogous forms, which have been referred to this genus. The Atrypa hemispherica 

 of MuRCHisoN is apparently a characteristic form of this genus, as well as the A. 

 planoconvexa, both of the Clinton group*. The internal structure, however, has been 

 determined from the species of the Oriskany sandstone and Lower Helderberg 

 specimens, but mainly from the former. 



In its hinge structure it approaches Tropidoleptus; but the hinge-line of the 

 dorsal valve is never so far produced, while the beak is much more extended. In 

 Tropidoleptus there is a distinct linear area, and the teeth and sockets are strongly 

 crenulated ; the form and character of the muscular impressions are different, and 

 the structure of the shell is punctate. 



Geological range. The genus begins its existence as low down in the system as 

 the Clinton gmup, and extends through all the members of the series to tlie lime- 

 stones of the Upper Helderberg, and perhaps above that point. 



• The latt-namod species presents some diflTerences in tlic hinge structure, wliich may prove of suflicjjut 

 impurtanco to unite the lower group of species under another generic designation. 



