EXTINCT AMPHIBIA OF NORTH AMERICA 45 
rib lying between the fifth and sixth vertebrae but to which it belongs 
is uncertain. 
There are impressions of ten ribs preserved on one side of the 
vertebral column and one on the other side. They are short, straight, 
and heavy as are the same elements in Branchiosaurus. ‘This char- 
acter alone is sufficient to place Micrer peton among the Branchiosauria 
since no such ribs are known in other groups of the Stegocephala. 
The ribs preserved lie next the seventh to the seventeenth vertebrae 
on the left side and there is one on the right side which may belong to 
either the fifth or sixth vertebra. ‘The ribs are central in their attach- 
ment and in this they agree well with the mode of rib attachment of 
the ribs in modern salamanders. All of the ribs are single headed 
and are composed, for the most part, of perichondral tissue. The 
position of the ribs in the matrix, inclined backwards and making a 
small angle with the vertebral column is very suggestive of the condi- 
tion found in Branchiosaurus. 
The pectoral girdle is represented by three distinct elements of 
the left side. ‘They are identified as scapula, clavicle, and coracoid. 
This is the nomenclature given by Woodward although Credner 
would call them otherwise. The nomenclature and morphology of 
the élements of the pectoral girdle will be discussed fully elsewhere and 
is not necessary here. The scapula is represented by an ovoid frag- 
ment lying next to the vertebral column. The clavicle was probably 
spatulate as it is in Melanerpeton but the inner end of the element 
is not visible. The coracoid is represented by its outer end only and 
its inner pointed extremity is not visible. The interclavicle has not 
been detected. 
The humerus lies somewhat to one side of the pectoral girdle as 
if there had been a large amount of articular cartilage. Its position 
may be due to post-mortem shifting but there is little other evidence 
of any movement after deposition. The humerus is a large heavy 
bone in comparison to the rest of the skeleton. It is expanded at 
each end and its ends show concavities proving that the bone is 
formed principally of perichondral tissue as would be expected from 
such an early Branchiosaurian. The endochondrium has not yet 
developed in this form which is evidently adult. There is no other 
element of the arm present. 
