MUSEUM OF COMPARATIVE ZOOLOGY. 



61 



M. dolichoceius and M. acer. The inner contour of the uorns 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 M.platyceras. 4. Viewed from in front and in section. 



4«. 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. 



The above measurements bring out very clearly tne 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 M. 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. XIIT. 11 



