AND AVES OF NORTH AMERICA. 87 



Iguanoclon and Hadrosaurus there is a very small sphaeroid condyle on the inner side of 

 a broad proximal extremity. If the condyle only articulated with the scapula, the rota- 

 tion of the humerus would be very limited; if the long narrow proximal articular surface, 

 which is the whole of the flattened proximal extremity of this bone the rotation would be 

 still less. In Laelaps however I find no round condyle, only the long narrow articular 

 face of the proximal extremity, as in the Crocodiles. This would not allow of abduction 

 and adduction, but as in the bird, of only flexure and extension. This is readily seen in 

 the movements of the Crocodile. I suppose the anterior limbs were more useful as sup- 

 ports when these animals placed the head near the ground, than for any other purpose, 

 especially in Laelaps and its allies. 



The character of the articulation of the vertebral column by intervertebral discs, the 

 double headed ribs, the elongate sacrum and large medullary cavities of the long bones 

 have been cited by Prof. Owen in evidence of the Mammalian tendencies of the animals 

 of this subclass. Their reptilian features, the single occipital condyle, quadrate and 

 coracoid bones, with the median tarsal ginglymus, are equally shared by the Aves, though 

 most of the usual distinctions between the latter class and the Reptiles hold good here 

 also. Prof. Owen also points out a special bird-like tendency in the alternation instead 

 of superposition of the neural arches of the sacrum on their centra ; and other points can 

 can now be added. Thus the reduction of the metatarsals to three in some of the genera, 

 and their close approximation and excess of length over the phalanges, brings to mind 

 these bones in the penguin. With the same reduction follows the confluence of the first 

 series of the tarsal bones, and the great diminution of strength of the fibula and its close 

 application to the tibia ; the front limbs are much reduced, and the long bones more pneu- 

 matic. In the most extreme form in this direction known, the first series of tarsal bones 

 is entirely confluent with the tibia as in the birds, the three metatarsals are much elon- 

 gate, the cervical vertebrae increase in number, and the pubes assume a position at right 

 angles to the vertebral axis, which is intermediate between their anterior position in most 

 Reptiles, and their posterior, in Birds. 



These features indicate three perhaps suborders, which are defined below. 



Quite as important, as indicating the avine affinity and remarkable character of this 

 order, is the evidence derived from the pelvis. And first, the support of this arch, the 

 femur, has been already alluded to. The head and neck of this bone are at right angles 

 to the direction of the condyle. In other reptiles the axes of these are oblique to each 

 other, so that the femur does not move in the direction of the axis of the body, but. 

 obliquely to it, thus permitting the body to rest on the earth. In the present, case the 

 structure is the same as in the birds and Mammals ; the femur could only move in a plane 

 parallel with the axis of the body. The reduced length of the fore limbs of many Dino- 



