Masonoceras

Masonoceras is a genus of Karagondoceratids from Lower Mississippian strata, the shell of which is thinly subdiscoidal to discoidal with an acute ventral margin in late ontogeny. Whorls are strongly embracing, umbilicus narrow to occluded. The mature external suture contains a wide trifid ventral lobe, the flanking prongs longer that the medial, an asymmetrically rounded lateral saddle and a deeper asymmetric pointed lateral lobe. Internal molds of the type, Mesoceras Kentuckiense show presence of a broad hyponomic sinus flanked by high rounded vantrolateral salients.

Mesonoceras is similar in general form to tis probable ancestor Karagandoceras but differs primarily in its pointed asymmetric lateral lobe, that of Karagandoceras is more symmetric.