EVOLUTION OF THE VERTEBEATA. 333 



There are several groups which show special marks of degen- 

 eracy. Such are the reduced maxillary bones and persistent gills 

 of the Proteida ; the absence of the maxillary bones and the 

 presence of gills in the Trachystomata ; the loss of a pair of legs 

 and feebleness of the remaining pair in the sirens ; and the ex- 

 treme reduction of the limbs in Amphiuma. Such I must also 

 regard, with Lankester, the persistent branchise of the Siredons. 

 I may add that in the brain of the Proteid Necturus the hemi- 

 spheres are relatively larger than in the Anura, which are at the 

 end of the line. 



It must be concluded, then, that in many respects the Ba- 

 trachia have undergone degeneracy with the passage of time. 



VI. THE REPTILIAIS" LINE. 



As in the case of the Batrachia, the easiest way of obtaining a 

 general view of the history of this class is by throwing their prin- 

 cipal structural characters into a tabular form. As in the case of 

 that class, I commence with the oldest forms and end with the 

 latest in the order of time, which, as usual, corresponds with 

 the order of structure. I except from this the first order, the 

 Ichthyopterygia, which we do not know prior to the Triassic 

 period : * 



A. Extremities not differentiated in form beyond proximal segment. 



I. Os quadratum immovably articulated to squamosal, etc. 



Tubercular and capitular rib-articulations present and distinct. . .1. Ichthyojjterygia. 

 AA. Elements of extremities differentiated. 



II. Os quadratum immovably articulated ; capitular and tubercular rib-articula- 



tions distinct. Archosauria. 

 Pubis and ischium united, and with little or no obturator foramen ; one posterior 



cranial arch ; limbs ambulatory ; a procoracoid 2. Theromorpha. 



Ischium and pubis distinct, the latter directed forward, backward, or downward ; 

 two posterior cranial arches ; limbs ambulatory ; no procoracoid 



3. Dinosauria.\ 

 Ischium and pubis united ; two postcranial arches ; anterior limbs volant 



4. Ornithosauria. 



III. Os quadratum closely united to cranial arches ; but one rib-articulation. 

 Synaptosauria. 



* Generally similar to the system published by me in " Proceedings Amer. Ass. 

 Adv. Science," xix, p. 233. 



f This definition includes the Crocodilia in the Dinosauria, as it is absolutely 

 connected with the typical Dinosaurs by the Opisthoccela (Sauropoda Marsh). 



