CEPHALOPODA. 791 
Triptoceras planoconvexum.] 
Family HUDOCERATID At. 
This group was erected by Hyatt to include orthoceran shells having a greatly 
compressed form, broad lobes and narrow saddles, with transverse section fusiform 
or subtriangular. The family was designed by its author to embrace the genera 
Eudoceras, (Hall) Hyatt, Triptoceras, Hyatt, Hdaphoceras, Hyatt, and Hndolobus, Meek 
and Worthen. 
Genus TRIPTOCERAS, Hyatt, 1883. 
Compressed orthoceran shells with broad ventral and dorsal lobes and acute 
lateral saddles; sipho ventral. The shell may be slightly arcuate but = usually 
straight at maturity; in transverse section subtriangular. 
TRIPTOCERAS PLANOCONVEXUM /[Ta/l, 1861. 
PLATE LVI, FIG. 3; PLATE LVII, FIG. 1. 
Orthoceras planoconvexum HALL, 1861. Rept. Supt. Geol. Surv. Wis., p. 47. 
Orthoceras planoconverum WHITFIELD, 1882. Geology of Wisconsin, vol. iv, p. 228, pl. vu, fig. 14. 
Original description: “Shell of medium size, gradually expanding from the 
apex toward the outer chamber, plano-convex; transverse section semicircular or 
subtriangular, the diameters as five to nine. The convex side a little depressed on 
each side of the middle, the opposite side nearly flat, the edges abruptly rounded. 
Septa. moderately concave, arching upwards on the sides, somewhat closely arranged, 
about five in half an inch. Siphuncle small, central. A specimen of the outer 
chamber, apparently of this species, is a little more than two and a half inches in 
length, one inch and an eighth in width, the short diameter being half an inch; the 
septa are about one-tenth of an inch distant.” 
A rather small but characteristic example of this species presents the convex 
side exposing fifteen septa in a distance of one inch, the body chamber having about 
the same length, so faras exposed. The curvature of the septal lobes is perfectly 
regular and the junction of the septa with the lateral margins distinctly acute. A 
fragment of a much larger individual has a body chamber measuring 60 mm. in 
length and 53 mm. in width near the aperture. To this fragment are attached three 
air-chambers the last exposing a clean septal surface and showing the ventral position 
of the sipho. The specimen shows that while the lateral saddles appear to be acute 
when viewed from the dorsal side, they are actually somewhat obtuse, the obtuse- 
ness of the angle being distinctly manifested only on the ventral surface. A line 
drawn from one lateral angle to the other shows that the dorsal convexity of the 
shell is about twice the ventral. 
