STEGOCEPHALIA. 



135 



arch, the lower facette with the hypocentrum. The hinder ribs exhibit a 

 single articular facette, connected with the neural arch. The interclavicle 

 is rhomboidal (tig. 84 E), and the clavicles are triangular in form ; all 

 exhibit a radiating sculpture. Both coracoid and scapula are ossified. 

 The limbs are stout, with four digits in the anterior, and five digits in the 

 posterior pair. There are also ossifications in the carpus and tarsus. The 

 fore limb is only about half as large as the hind limb, and not so well 

 known as the latter. Seven carpal bones have been observed in one 

 specimen. The best-preserved tarsus hitherto discovered is shown of 

 one-half the natural size in fig. 87, and a plausible interpretation of it is 



FIG. 87. 



Archegosaurus decheni ; portion of hind limb, one-half nat. size. L. Permian ; 

 Rhenish Prussia. F, fibula ; x, tibia ; a, 6, doubtful ossifications ; c, cen- 

 tralia ; /, fibulare ; t, intermedium ; t, tibiale ; x, doubtful centrale ; 1 4, 

 tarsalia ; i iv, metatarsalia, one wanting. (After Baur.) 



indicated by the lettering. The number of phalanges in the digits of the 

 hind foot is determined to be 2, 2, 3, 4, 3. There is a ventral armour of 

 overlapping, transversely-elongated scutes ; and small irregular dermal 

 calcifications are scattered over the trunk and limbs. The typical species, 

 Archegosaurus decheni, attains a length of not less than 1'5 m., and is 

 found chiefly in the Lower Permian of Rhenish Prussia ; more frag- 

 mentary specimens are recorded from corresponding formations in Saxony 

 and Bohemia. 



Actinodon is closely similar to Archegosaurus, but has a 

 shorter and broader skull with an ossified basioccipital region. 



