6oo S. W. WILLISTON 



valid for them. With the ehmination of these genera, and perhaps 

 others, from the Amphibia there remain a number of others hitherto 

 classed under the Microsauria, of which Urocordylus, Crossotelos, 

 and perhaps Diplocaulus are typical, that are genuine stegocephalian 

 amphibians, which can no more be classed with the Reptilia than 

 a salamander can. They have exogenous chevrons, and double- 

 headed ribs attached to body and arch, true amphibian characters, 

 the former utterly unknown among reptiles, save apparently in 

 such rare cases as Clidastes among the mosasaurs, purely the 

 result of a secondary anchylosis. There have been plenty of 

 terms proposed to include them, such as Nectridea Miall, Lepo- 

 spondyli Zittel, Diplocauha Moodie, Holospondyli Jaekel. For 

 myself I prefer the term Lepospondyli. Jaekel's class Micro- 

 sauria is untenable. 



Whatever may be the final disposition of Lysorophus, which I 

 have referred to the order Urodela or Caudata, there can be hardly 

 a question of its urodelan affinities. It has nothing to do with the 

 Gymnophiona. The abundant material in the University collec- 

 tions demonstrates the presence of small limbs in numerous speci- 

 mens. Amphiuma-like in form it had Amphiuma-like habits and 

 limbs. I may also add that the supposed proatlas described both 

 by myself and Case is merely the arch of the so-called atlas. 



EXPLANATION OF PLATE 



Fig. I, Pleuristion brachycoelus Case, left humerus, ventral side; Fig. 2, 

 the same, outer side; Fig. 3, Sphenodon punctatus, left humerus, ventral side; 

 Fig. 4, Araeoscelis gracilis WUl., right humerus, inner side; Fig. 5, the same, 

 ventral side; Fig. 6, the same, distal end, dorsal side; Fig. 7, A. gracilis, left 

 femur of young individual, dorsal side; Fig. 8, the same, inner side; Fig. 9, A. 

 gracilis, adult femur, dorsal side; Fig. 10, the same, ventral side; Fig. ii, 

 A. gracilis, caudal vertebra, side view; Fig. 12, the same, from below; Fig. 

 13, ^. gracilis, dorsal vertebra, from side; Fig. 14, A. gracilis, upper part of 

 tibia; Fig. 15, A. gracilis, metapodial; Fig. 16, A. gracilis, phalanges in 

 position as found; Fig. i'], A. gracilis, dorsal vertebra; Fig. 18, A. gracilis, 

 sketch of skull; Fig. 19, A. gracilis, mandibular teeth. All figures natural 

 size, save where indicated. 



