STEGOCEPHALIA. 125 



correspond exactly with the posterior cheek-plate in Rhizo- 

 dopsis, Holoptychius, and their allies. This and the quadrato- 

 jugal are closely apposed to the quadrate, without any in- 

 tervening foramen or vacuity. The true supratemporals are 

 apparently represented in the two pairs of superficial bones 

 (s. t.) which complete the covering of the cranium behind and 

 are commonly described as supraoccipitals and epiotics. On 

 either side of this transverse row of plates there is a notch 

 in the hinder border of the cranial roof, of variable extent, 

 evidently for the opening of the auditory meatus. The chon- 

 drocranium itself usually exhibits very imperfect ossification, 

 and this is first observable in the basioccipital region, where 

 the skull is articulated with the vertebral axis. The chief 

 features of the palate are the large size of the palatine vacuities 

 and the enormous development of a parasphenoid membrane- 

 bone. 



The whole of the cheek being covered with plates, these 

 early lung-breathers are commonly grouped together under the 

 name of STEGOCEPHALIA (roofed-head) ; and three chief reasons 

 may be mentioned for regarding them as representatives of 

 the Class Batrachia. Traces of gill-arches are conspicuous in 

 immature skeletons of some genera. The ribs, so far as known, 

 do not encircle the thorax or show any indications of connection 

 with sternal elements. The mucus-canal system must have 

 been especially well developed, as indicated by deep impres- 

 sions frequently observed on the head. 



The Stegocephalia first appear in the Lower Carboniferous 

 of Western Europe, but these early representatives of the order 

 are still very imperfectly known. The want of all information 

 as to their limbs is particularly unfortunate, since every known 

 limb of Upper Carboniferous and later date conforms to the 

 normal pentadactyle type, with no approximation to the paired 

 appendages of fishes. All (or nearly all) the genera, however, 

 agree in possessing one median and two paired exoskeletal 

 plates in the thoracic region between the pectoral limbs ; these 

 being commonly interpreted as interclavicle and clavicles. 

 There are also very few exceptions to the rule, that a ventral 

 armour of small overlapping scales or scutes is present behind 



