Type: M. longicaudatum Moodie.
The genus is distinguished from other known branchiosaurian genera by the great length of the dorsal region, the elongate tail ([plate 5, fig. 2]), with its well-developed caudal ribs, the reduction of the tympanic notch, the broad nature of the scapula, the elongate interclavicle, and the slender ilium. The number of dorsal vertebræ is identical with that of Branchiosaurus of Saxony.
Mazonerpeton longicaudatum Moodie.
Kans. Univ. Sci. Bull., VI, No. 2, p. 337, pl. 3, figs. 1-2; pl. 7, fig. 3; pl. 10. 1912.
Type: Specimen No. 795 (1234), Yale University Museum.
Horizon and locality: Mazon Creek shales, near Morris, Illinois. ([Plate 3, figs. 5 and 6.])
The remains consist of the following elements: an incomplete skull; nearly the entire vertebral column, consisting of cervical, dorsal, sacral, and caudal vertebræ, 36 in number; several ribs preserved on each side of the vertebral column; a portion of the ventral armature; the scapulæ; a clavicle; the interclavicle; both humeri; the radius and ulna of one side and the ulna of the other; portions of both hands; the ilium of the right side; both femora, and a partial impression of the left tibia.
The skull is, unfortunately, very poorly preserved. Enough remains, however, to determine the essential characters. The skull bones, unlike any other American branchiosaurian, have an ornamentation consisting of sharp pits and elevations which in places have a quincuncial arrangement and in others take the form of definite lines of pits or tubercles similar to the condition found in many of the Microsauria. The orbits are large and are situated back of the median transverse line of the skull. They are almost circular in form and contain 6 elongated sclerotic plates very closely arranged around the borders of the right orbit. The plates are twice as long as wide. The interorbital width is 1.25 times the transverse diameter of the orbit.
Not many of the sutures of the skull are discernible. Portions of the frontals, the nasals, the prefrontals, the parietals, and the supratemporals can be identified. Their arrangement is shown in [figure 14a]. There is a decided posterior table to the skull, with truncate posterior border. The tympanic notch is shallow, with its outer border not so well protected as in Branchiosaurus.
The cervical vertebræ are incomplete, but their number was 4 or 5, as in Micrerpeton. The structure of the dorsal vertebræ is also uncertain, although the shape can be discerned. The vertebræ are short and thick, very unlike the long, cylindrical vertebræ of Cephalerpeton. The heavy transverse process is quite evident on the best preserved vertebræ. This process recalls that described by Credner for the Saxony Branchiosauria. Several of the vertebræ show the articulation of the ribs with this process. The ribs of the caudal region recall very strongly those of Branchiosaurus. They are quite heavy in the anterior caudal region and then diminish rather rapidly to the point where the tail is broken and lost.