STEGOCEPHALIA. 133 



The large and typical Labyrinthodonts of the Coal Measures 

 are also known only by fragmentary specimens, chiefly skulls or 

 portions thereof. In these the basioccipital region always 

 appears to be ossified, and the vertebral axis varies from the 

 rhachitomous condition (see p. 94) to a chain of complete, 

 biconcave discs. The skull of Loxomma exhibits a great 

 antorbital vacuity not separated from the orbit itself; and the 

 grooves for the mucus-canals are especially prominent. Its 

 teeth are laterally compressed. The skull of Anthracosaurus, 

 which sometimes attains a length of nearly half a metre, is 

 broader than that of Loxomma, with normal orbit, and round 

 teeth. The vertebral centra in this genus are biconcave discs, 

 very slightly pierced by the notochord ; the thoracic ribs are 

 double-headed. 



The Lower Permian Labyrinthodonts are known by many 

 tolerably well-preserved skeletons, the best being those of 



/] Archegosaurus (figs. 84 E, 86, 87). The skull in this genus is much longer 

 ' than broad, and the facial region elongates with age. The orbits are 

 situated in its hinder portion, and there is a ring of sclerotic plates. The 

 external bones are sculptured, but the course of the mucus-canals is very 

 feebly marked. The basioccipital region is not ossified, and the palatine 

 vacuities are very large. The parasphenoid and pterygoids are toothless, and 

 the vomers bear only one or two pairs of relatively large teeth ; but there 

 is a single series of teeth on the maxillae and premaxillee, and another 

 along each of the palatines. The latter element lies extended parallel to 

 the maxilla on each side, and enters the hinder margin of the posterior 

 nares, which are otherwise bounded completely by the vomers. The 

 mandible (fig. 86 c) exhibits on its outer face a lower marginal element 

 (i.d.), which may be either the exposed border of the splenial or a 

 distinct infradentary bone. Branchial arches with denticles have been 

 observed in immature individuals. The notochord is shown to have been 

 persistent, encircled throughout its length by paired pleurocentra and 

 median bilaterally-symmetrical hypocentra. In the presacral region, 

 which comprises about 25 vertebrae, the pleurocentra are normal ; but 

 in the anterior part of the caudal region there is a supplementary pair 

 of smaller cartilages below the ordinary pleurocentra of each vertebra. 

 The sacrum seems to include the elements of only one vertebra. There are 

 well-developed neural arches united by zygapophyses ; and in the caudal 

 region there are chevron bones fused with the hypocentra. Ribs are 

 present throughout the trunk and in the front portion of the tail. All are 

 expanded at the extremities, and the foremost are articulated by a double 

 head, the upper facette with the short transverse process of the neural 



