24 THE COAL MEASURES AMPHIBIA OF NORTH AMERICA. 



the group. Judging from Credner's studies on the series of specimens of Branchio- 

 saurus amblystomus Credner (187), the skull bones do not ossify completely until 

 relatively late in the life of the individual. The skull in the youngest individual 

 figured by Credner (op. cit., Taf. xvi, fig. i) seems to be largely cartilaginous, with 

 beginnings of separation into the skeletal elements. The manner and time of devel- 

 opment and ossification of the skull seems to proceed much as it does in modern 

 amphibians. The condition found in the skull of Cryptobranchus allegheniensis or 

 Necturus maculosus will represent pretty accurately the condition of most of the 

 Coal Measures Amphibia. The face bones in certain forms were sculptured and 

 cut by lateral-line canals. 



A median suture divides the skull into two equal regions dorsally. On the sides 

 of this median suture lie pairs of elements which are common to all higher verte- 

 brates. These elements are: the premaxillae, nasals, frontals, parietals, and post- 

 parietals. All of these elements vary somewhat in shape and slightly in arrange- 

 ment, but always occupy the same relative positions. To the side of these elements 

 lie the prefrontal, the postfrontal, the supratemporal, the squamosal, and tabulare, 

 and occupying the margin of the skull are the maxilla, the jugal, the quadratojugal, 

 and possibly the quadrate in a few forms. The parietal foramen occurs usually 

 within the parietal bone, but its position is subject to slight variations and it may 

 occur on the suture between the frontal and the parietal, or even far posterior near the 

 postparietal. The nostrils often lie well forward and are included by the premaxilla?, 

 nasals, and prefrontals. The orbit is usually well posterior, but it may occur far 

 forward. It is bounded by the prefrontal, the frontal, the postfrontal, the post- 

 orbital, and the jugal. Sometimes the lacrimal is present and has been clearly 

 identified on the anterior margin of the orbit in a few cases. 



(b) Sclerotic plates often occur within the orbits, and are not confined to any par- 

 ticular group, though they are quite constant among the Branchiosauria. They are 

 usually delicate, thin, broad plates which evidently overlap and operate as in mod- 

 ern animals. The number varies, as many as 30 occurring within the orbit of one 

 branchiosaur. Between the margin of the orbit and the sclerotic plates there often 

 occur, in the Branchiosauria (186) particularly, small scale-like particles which 

 were doubtless embedded in the heavy skin above the orbit during life. 



(c) The palate of the skull is very incompletely known, being indicated in a very 

 few cases. These specimens, however, show that the characters of the palate were 

 quite similar, if not identical, in essential respects with the palate among the Euro- 

 pean species of the same or slightly later time. 



A large cultriform parasphenoid occupies the posterior portion of the palate, on 

 either side of which in some species lies the posterior palatine foramen. On the 

 sides of the anterior prolongation of the parasphenoid lie the vomers (186), membra- 

 nous bones often bearing minute tubercular teeth, apparently adapted for crushing. 

 The vomers and the maxillae, with sometimes the palatine, surround the anterior 

 palatine foramen, which is almost always present; sometimes, however, quite small. 

 The transverse or ectopterygoid unites the pterygoid, a broad plate of thin bone, 

 with the maxilla and jugal. 



