54 THE COAL MEASURES AMPHIBIA OF NORTH AMERICA. 



is elongate and forms the antero-lateral border of the skull. The jugal forms an 

 important element in the lateral border of the cranium and joins the quadratojugal 

 jX)steriorly. The postfrontal is triangular and with the postorbital forms the pos- 

 terior border of the orbit. Both of the elements are acuminate posteriorly, although 

 the suture between them is indistinct, and they inclose between their posterior acumi- 

 nations an anterior projection of the supra temporal. The squamosal has the usual 

 relations and borders the supra temporal laterally. The latter element forms the 

 quadrate angle of the cranium. 



The entire length of the vertebral column is preserved, although the nature and 

 structure of its elements can not be determined. The impressions of a few of the 

 vertebrae show that some of the centra were amphicoelous, but other than this 

 nothing is definite. The cavities which the centra occupied were filled by the white 

 mineral matter and the force of the blow which cracked the nodule destroyed the 

 form of the mold. It is possible that where the mineral matter has filled the cavities 

 the centra were osseous or partly cartilaginous, and where the cavities were unfilled 

 the centra were entirely cartilaginous. The length of the vertebral column from the 

 base of the skull to the last impression of a cartilaginous centrum is 33 mm. 



The number of centra between the sacral vertebra and the skull is 20 as they 

 are preserved, but there may have been one more, the atlas. Fritsch has represented 

 21 in his restoration of Branchiosaurus salamandroides, and this is a further indica- 

 tion of an affinity between the two genera, although Credner has represented 26 pre- 

 sacral vertebrae in Branchiosaurus amblystomiis. The presacral vertebrae are thus 

 seen to vary within narrow limits, but the number of presacrals is near 20, and this 

 may be taken as typical. It is interesting to notice that in modern forms of the 

 salamanders the presacral vertebrae number about 20. There is but a single sacral 

 centrum in Micrerpeton. The sacral rib has not been detected, but it is restored 

 after the condition found in Branchiosaurus. The right femur partially covers the 

 sacral vertebra, and its structure can not be determined. I count impressions of 17 

 caudal centra, of which at least 12 may have been partially ossified. In the cervi- 

 cal region there are distinct impressions of transverse processes on at least 5 verte- 

 brae, and this number is assigned to the neck, although it is by no means certain 

 that this is the correct number. The neck was at least short, if we may judge from 

 the position of the remains of the pectoral girdle. No cervical ribs are definitely 

 determined. There is a short rib lying between the fifth and sixth vertebrae, but to 

 which it belongs is vmcertain. 



There are impressions of 10 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 ele- 

 ments in Branchiosaurus. This character alone is sufficient to place Micrerpeton 

 among the Branchiosauria, since no such ribs are known in other groups of the 

 extinct amphibians. 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. They are central in their attachment, and in this they 

 agree well with the mode of rib attachment in the modem salamanders. All of the 

 ribs are single-headed and are composed, for the most part, of perichondral tissue. 



