MUSEUM OF COMPARATIVE ZOOLOGY. 161 
M. dolichoceras and M. acer. The inner contour of the horns is concave; they 
are greatly flattened antero-posteriorly with a ridge-like outer margin, and 
connected by a well-raised median ridge. The posterior face is nearly plane, 
Figure 4.— Horns of MV. platyceras. 4. Viewed from in front and in section. 
4a, Viewed from the side. 
the anterior is convex, so that the section of the horn is plano-convex from 
base to tip. In side view the horns completely overhang the nasals, and are 
slightly recurved. The long axis of the horn section is directly transverse. 
MEASUREMENTS. 
Free portion Approximate 
of diameter of 
nasals, anterior nares. 
outside 
Verti- | Trans- 
cal. verse. 
nasal tips, 
tween horns. 
of horns, 
measurement, 
ured from tips to 
median fronto-nasal 
ameter of the molar- 
premolar series. 
Length. | Breadth. 
Occipital condyles to 
Nasal tips to ridge be- 
Distance between tips 
Length of horns meas- 
Antero-posterior di- 
M. 
M. coloradensis 70 | .16 33 1 31 08 2 078 185 
M. tichoceras 802) . pas Vee 3 4 : : ‘ .09 
M. dolichoceras 
M. platyceras 
M. Proutii 
The above measurements bring out very clearly the decrease in the propor- 
tions of the nasals pari passu with the gradual elongation of the horns. An- 
other very interesting fact is brought out by the comparison of the transverse 
and longitudinal diameters of the horns at the base. As we pass from the 
short to the long horned types, through W. coloradensis, tichoceras, dolichoceras, 
and platyceras, there is a gradual rotation of the longer axis of the horn-section 
from a fore and aft to a transverse plane; the species last named representing 
the extreme of the transverse type. 
VOL. XIII. — No. 5. 11 
