302 
PALAEONTOLOGY OF NEW YORK. 
Orthoceras Thestor, n. sp. 
PLATE LXXXII, PIG. 18. 
Shell straight, regularly and gradually enlarging. Transverse section circular. 
The characters of the chamber of habitation, air-chambers, septa and 
siphuncle are unknown. 
Tube ornamented with very numerous, sharp annulations. The annula- 
tions are straight and horizontal, and about twelve in the space of ten mm. 
Surface marked by regular, fine, longitudinal striae, crossed in the furrows 
between the annulations by sharp, transverse striae. The longitudinal striae, 
of which there are ten in the space of two mm., are continuous over the 
annulations, crenulating their summits. 
This species is distinguished by the extreme frequency and regularity of the 
annulations, in which it differs from the other annulated forms. The surface 
characters are also different from those in 0. lima. In 0. Idmon the annulations 
are more abruptly elevated and less frequent; being as seven to twelve in the 
two species, with the intermediate furrows flattened. 
Formation and locality. From the carbonaceous beds of the Marcellus shales 
of the Hamilton group at Schoharie, N. Y. 
Orthoceras Idmon, n. sp. 
PLATE XLIII, FIGS. 11, 12. 
Shell straight, regularly enlarging. Transverse section, chamber of habitation, 
septa and siphuncle not observed. 
The air-chambers are regular, as frequent as the annulations, or having a 
depth of 1.5 mm. Sutures straight and horizontal. 
Test thin. Tube ornamented by numerous, regular, abruptly elevated 
annulations, with flattened interspaces; of which there are seven in the 
space of ten mm. Surface marked by regular, fine, continuous, longitudinal 
striae, with concentric lines of growth in the furrows. 
The specimen figured has a length of forty mm., and preserves twenty- 
five annulations in this space. 
