MOODIE: MAZON CREEK, ILL., SHALES. 341 
The type is specimen No. 795 (1234), with obverse, of the Yale 
University Museum. Collected at Mazon Creek, Grundy county, Illinois. 
Mazonerpeton costatum new species. 
(Plate 2, fig. 3; plate 8, fig. 4; plate 9, fig. 2; plate 10.) 
The remains on which the present species is based are in- 
closed in a much-fractured nodule. The parts of the animal 
which have been identified are as follows: A part of the skull 
and left mandible, two clavicles, a humerus, impressions of 
several vertebre, a portion of the dorsal region of the body, 
with several ribs, two portions of the caudal region, with 
several ribs, and some unidentified fragments. 
The animal, from the shape and form of the ribs, is un- 
doubtedly a representative of the Branchiosauria, since short, 
heavy, straight ribs have not yet been found to be associated 
with other than branchiosaurian structures. Its association 
in the same genus with Mazonerpeton longicaudatum is held 
to be correct, on account of the resemblance in structure of the 
pectoral elements, the form of the humerus, and the length of 
the tail. The present species is about one-half larger than 
Mazonerpeton longicaudatum, and the animal which repre- 
sents the species perhaps attained a length of four and one- 
half inches, while the length attained by the type of M. longi- 
caudatum was not more than three inches. The tail of the 
present species is very long and slender, more elongate than in 
any other described branchiosaurian. 
The part of the skull preserved is very unsatisfactory, and, 
aside from the fact that it seems to represent the under side 
of the left half of the skull, little can be said. Portions of 
three sutures can be observed, but what sutures they are is 
undetermined. The left mandible lies crushed on the edge of 
the skull and partially obscures what little there is of that 
structure. The slightly curved impression, from which the 
bone has been either broken or weathered, measures thirteen 
millimeters in length by three in posterior diameter by one in 
anterior diameter. These measurements show the element to 
have been slender and pointed anteriorly. 
Very little accurate information can be derived from the 
study of the vertebral column of the specimen. The dorsal 
vertebral formula can not be made out, since only a portion of 
the length of that region is preserved, and only a few rather 
indefinite impressions can be discerned. These impressions 
