CEPHALOPODA. 
33 
Gomphoceras crenatum. 
PLATE CXXI A, FIG. 2. 
Gomphoceras crenatum, Hall. P'ifth Ann. Kept. State Geologist. Expl. pi. (1-21 a) 6, fig. 2. 1886. 
This species is based upon a specimen consisting of the chamber of habita¬ 
tion and a portion of ten attached air-chambers. The point of greatest trans¬ 
verse section is at about the second septum from the outer chamber. 
Chamber of habitation a little longer than the transverse diameter, gradually 
contracting to the aperture. The base of this chamber is marked by a zone of 
strong elevated lines, distant from each other about 4 mm. and extending over 
the walls of the air-chambers. Air-chambers about 7 mm. in depth in the 
main body of the tube. 
The specimen described has a length of 121 mm., of which 50 mm. pertain 
to the chamber of habitation. The greatest transverse diameter of the tube 
measures 56 mm. 
Distribution. In the limestone of the Upper Helderberg group, Columbus, 
Ohio. 
Gomphoceras plenum. 
PLATE CXXI A, FIGS. 3, 4. 
Gomphoceras plenum, Hall. Fifth Ann. Kept. State Geologist. Expl. pi. (121 a) 6, figs. 3, 4. 1886. 
Shell large, stout, sub-fusiform. Transverse section circular ; point of greatest 
gibbosity at about the tenth septum from the chamber of habitation. Axis 
nearly straight. Tube rapidly enlarging to the point of greatest transverse 
section, more gradually contracting to the outer third of the grand chamber, 
and abruptly constricted at the aperture. 
Chamber of habitation as long as the diameter of the tube at the last 
septum. Aperture obscurely trilobate. Air-chambers from 6 to 7 mm. in 
depth over the larger portion of the shell; the last three being shallower. 
Sutures slightly curved. 
Siphuncle situated near the ventral side of the tube; its elements in the 
interseptal spaces have not been observed. 
