CHAP. IV STEGOCEPHALI 79 



forming a suture, along the lateral side of the vertebra, both 

 partaking in the formation of a transverse process which carries 

 the rib. 



2a. Temnospondylous. The vertebra is composed of three 

 pairs of units, which remain in a separate, unfused state. 

 Two of them are dorsal arcualia, one of which tends to form the 

 centrum of the vertebra, which then carries the neural arch. 



2b. Stereospondylous. The three component units fuse by 

 co-ossification into a solid, amphicoelous vertebra. 



The ribs are one- or two-headed, rather strong, but short, 

 rarely reaching half-way round the body. They occur on all the 

 vertebrae of the trunk and on most of those of the tail. One 

 pair of ribs connects one vertebra, the sacral, with the pelvis, of 

 which the ilium and ischium are generally ossified, rarely also 

 a portion of the pubic region. 



The shoulder-girdle is very primitive, greatly resembling that of 

 , the Crossopterygian fishes. It consists of the following bones : 

 a median, rhombic, or T-shaped interclavicle, a pair of clavicles, 

 of cleithra, of coracoids, and of scapulae. The limbs show the 

 typical pentadactyle plan, but even in these earliest Tetrapoda 

 the hand possesses only four fingers, with 2, 2, 3, 2 phalanges 

 respectively. The foot has five toes, with 2, 3, 4, 4, 3, or 

 2, 2, 3, 4, 3 phalanges. 



Many Stegocephali were possessed of a dermal armour, covering 

 either the whole body or only the under parts. Hence the term 

 Phractamphibia (^>paT09, armoured). The armour consists of a 

 great number of small cutaneous scales, partly calcified, or 

 perhaps ossified, and arranged in many more or less transverse 

 rows. We can only surmise that these scales were covered by 

 corresponding epidermal sheaths. The skull is ideally complete 

 in the number of separate bones which appear on its surface. 

 Besides the outer nares and the orbits there is always an 

 unpaired, small, interparietal foramen.' The whole temporal 

 region is completely roofed over. The following bones are 

 present : nasals, frontaLs, parietals, supra- and latero-occipitals ; 

 lacrymals (unless fused with the jugals ?), prefrontals, postfrontals, 

 postorbitals, squamosals, and epi- (or opisth-) otics ; premaxillaries, 

 maxillaries, jugals, quadrato-jugals, and supra-temporals ; quad- 

 rates, pterygoids, palatines, vomers, and an unpaired para- 

 sphenoid. The lower jaw is composed of a pair of dentaries, 



