MICROSAURI 289 



as the Proreptilia — it is not to be wondered at that they still 

 retain a number of amphibian features. The skull is encased in 

 dermal bones as in the Stegocephali, and the dermal armour of 

 the trunk and tail is composed of many bony, sculptured scales, 

 which cover back, sides, and under surface. The middle rows on 

 the back are the largest, while the scales on the belly are 

 arranged in transverse rows, which imbricate and converge 

 obliquely headwards. Special gular plates seem to be absent. 

 The skull has an interparietal foramen. The jaws and the palate 

 are furnished with small, simple teeth, and there is a large 

 parasphenoid bone, an eminently amphibian character. The 

 occipital condylar articulation is supposed to be double. The 

 centra of the vertebrae are deeply amphicoelous, elongated, and 

 constricted in the middle, just like those of the Aistopoda 

 and Branchiosauri. The dorsal spinous processes are strongly 

 developed, and with the zygapophyses are very reptilian. Trans- 

 verse processes are absent or very short, the tubercular portions 

 of the ribs articulating with the centra, the capitula mostly 

 intervertebrally, in any case close to the anterior end of the centra. 

 The tail - vertebrae possess very typical, movable chevrons, 

 placed intervertebrally, and bear an extraordinary resemblance 

 to those of Geckos. The ribs are long and slender, but there is 

 no sternum. The fore- and hind-limbs are pentadactyle, in 

 opposition to the invariably four-fingered Stegocephali. The 

 shoulder-girdle consists of scapulae, coracoids, clavicles, cleithra, 

 and a T-shaped interclavicle. The pelvis also resembles that of 

 certain Stegocephali by the separately ossified, somewhat disc- 

 shaped, flat ischia and pubes, which seem to have been joined 

 together by cartilage into one broad mass. 



Hylonomus, Dawson's type of Microsauri, was found in the 

 Coal-measures of Nova Scotia, within decayed tree-stumps. Closely 

 allied, if not identical, but much better known is Hylo'plesion, e.g. 

 H. lo7igicostatum of the uppermost Permian of Nyrschan in 

 Bohemia. Total length under 4 inches ; eyes with bony sclerotic 

 rings ; neck- short. The truly Permian genera Dawsonia, Melan- 

 crpeton, Ortliocosta, and Seeleya are allied forms, the last scarcely 

 one inch in length, but well preserved. Petrohates of the 

 Triassic Lower Ked Sandstone of Saxony has an arrangement of 

 the ventral dermal armour closely resembling abdominal ribs. 



VOL. VIII u 



