78 FIFTEENTH REPORT ON THE CABINET OF NAT. HISTORY. 



ORTHOCERAS EXILE ( n. s.). ^J ' ^^ 

 Shell elongate, cylindrical, very gradually tapering, broadly and 

 gently constricted near the aperture, and swelling between that 

 point and the commencement of the septa : aperture a little 

 expanded ; septa distant, somewhat less than one-third the dia- 

 meter, very convex. Siphuncle small, a little excentric. 

 Surface transversely striate. 



This fossil is often mistaken for the O. constrictum; but if we are to 

 take the figure of Mr. Vanuxem, with the abrupt constriction and closely 

 arranged septa, that name is applicable to the preceding species. In the 

 present one the constriction is always gentle, and often extends over the 

 space of three-fourths of an inch ; the septa are much more distant, and the 

 siphuncle larger and always excentric. 



A specimen of this species, measuring nearly six inches long, has a length 

 of the outer chamber of two inches, and a diameter, when not compressed, 

 of about half an inch. In the first inch from the outer. chamber, the septate 

 portion has six chambers (not counting the narrow space next the outer 

 cavity) ; the second inch has seven chambers, and the third has nine. A 

 specimen from Cumberland (Maryland), Vrith essentially the same charac- 

 ters, has five chambers in the length of an inch where the diameter is five- 

 eighths of an inch. In numerous specimens of about the same proportions 

 and character, the siphuncle is excentric. 



Geological formation and locality. In the coarser shales of the Hamil- 

 ton group at Cazenovia (New-York), and near Cumberland (Maryland). 



ORTHOCERAS CROTALUM ( n. s.). ? /. /, 1^ 

 Shell cylindrical, somewhat rapidly attenuate : septa numerous, 



moderately convex. Siphuncle central or subcentral. 

 Surface marked by strong annulations, which are a little undula- 

 ting or bent backwards on one side, so as not to correspond with 

 the line of the septa ; these are crossed by fine even longitudinal 

 striae, wiiich are sometimes slightly undulated in passing over the 

 annulations. 



A specimen of a little more than half an inch in diameter, has seven or 

 eight septa in the length of an inch. The annulations cross the shell in such 

 a manner that nearly one half the circumference is upon one division or 

 chamber, and a little more than half the circumference on the next lower 

 one, and sometimes slightly affecting the next one below this. One, two, or 

 three narrow chambers occur at the beginning of the septate portion, or 



[ September, 



