TARSO-METATARSUS OP HESPERORNIS. 95 



Measurements. (No. 1477.) 



Anteroposterior diameter of proximal end of fibula, - 21.0 mm 



Transverse diameter of proximal end, 13.0 



Greatest diameter of shaft, 16.0 



Transverse diameter of shaft, at same point, 7.0 



The Taeso-Metataesus. (Plates XVI and XVII.) 

 In the tarsometatarsal bone of Hesperornis regalis, we have the 

 extreme modification of a form characteristic of modem diving' birds. 

 There were four digits in the foot, the fifth being entirely absent, as in all 

 known birds, recent and fossil. In the adult Hesperornis, the second, third, 

 and fourth metatarsals are thoroughly coossified into a stout, transversely 

 compressed bone of moderate length, but in most specimens traces of the 

 sutures remain. The fourth metatarsal element so greatly exceeds the 

 other two in size, that it fomis by far the greatest part of the entire tarso- 

 metatarsal bone. 



The plane of motion for the whole limb, which above was coincident 

 with the axes of the femur and tibia, was continued below through this 

 element of the tarso-metatarsal, and down through the fourth, or outer, 

 digit, which it supported. In this structure of Hesperornis, so admirably 

 adapted for swimming, we have an example of the same kind of specialized 

 modification which has prepared the foot of the Ostrich, among recent 

 birds, and the Horse among mammals, for extreme speed on the land. 



The proximal end of the tarso-metatarsal is covered by two articular 

 faces for union with the condyles of the tibia. The outer one is about half 

 the size of the inner, and the two are separated by a prominent obtuse 

 ridge. There is no hypotarsus, and there are no canals, or even grooves, 

 for tendons on the posterior face of the proximal end, as seen in the Divers, 

 and most other birds. This part of the bone is occupied by a rugose 

 triangular tract, covered in life by cartilage, over which the tendons 

 passed. Below this, there is a broad shallow depression, extending rather 

 more than half the way to the distal extremity. 



The outer surface of the tarso-metatarsus is slightly concave longi- 

 tudinally, and convex transversely. On the anterior face, there is a 



