FROM THE KIMMERIDGE CLAY. 4£3 



In a communication to the Geological Society, in 1869, Prof. 

 Huxley demonstrated the agreement of the Dinosaurian astragalus, 

 which he had shortly before identified in Megalosaurus, with the 

 distal condylar element of the bird's tibia, which Gegenbaur had 

 shown to comprise the equivalent of the astragalus in extant 

 reptiles. 



In most birds, as is well known, the individual distinctness of the 

 tibio-tarsal elements is soon lost by the acerescence of the tarsal 

 part to the tibia, their individuality continuing longest in the 

 keelless birds, of which the Ajpteryx furnishes an admirable example. 

 In Dinosauria, Prof. Huxley showed that the astragalus remained a 

 distinct part throughout the whole life. In the bird, as remarked 

 by Gegenbaur, the combination of two tarsal elements in the single 

 bone which is regarded as the equivalent of the astragalus, is 

 hinted by the facility with which, at an early embryonic stage, the 

 bone sometimes separates into two pieces in the attempt to detach 

 it from the tibia. In this Cumnor Iguanodon, and also in the 

 Wealden Iguanodon and in HypsilopJiodon, the distinctness of the 

 calcaneum is clearly preserved throughout life. It obtains also in 

 Megalosaurus, and it may fairly rank as a Dinosaurian character. 

 In Dinosauria the astragahi^ and calcaneum together are the homo- 

 logue of the astragalus of the young bird. 



In the grown bird the relation of the fibula to the calcaneum is not 

 apparent ; but it is otherwise in early embryonic existence, when 

 the fibula and tibia are of equal length, and the distal end of the 

 former reaches the mass of tissue out of which the proximal tarsal 

 element is afterwards evolved. In the Iguanodont foot these early 

 developmental phases are, as it were, permanently fixed. 



Amongst the Cumnor fossils there are none which I can identify 

 as elements of a distal tarsal row ; and from the study of two hind 

 feet of Iguanodon which I dug out in the Isle of "Wight, and also 

 of several feet of the allied HypsilojpJiodon, I suspect that such 

 elements, if they ever had a distinct existence, soon lost it by fusion 

 with the basal ends of the metatarsals. This coalescence of distal 

 tarsal elements with the metatarsus is foreshadowed in Compso- 

 gnathus, in which the former are represented by thin inconspicuous 

 disks united to the metatarsals, a narrow line marking their 

 primitive separateness. In Iguanodon probably such separateness 

 is restricted to the embryo. 



Metatarsus. — Parts of four bones are referable to this segment 

 of the foot. Nos. iv. 24, 25, 26, are the distal trochlear ends 

 probably of the two lateral and the middle metatarsals of the right 

 foot; iv. 27 is the trochlea of the left middle metatarsal. N^os. 33, 

 35, 36, are fragments of their proximal ends and shafts. 



The trochlea is wide and shallow. In the middle metatarsal it 

 reaches higher on the front of the joint than in the lateral meta- 

 tarsals, and the outer is rather stouter than the inner condyle. In 

 the lateral metatarsals the condylar division, less marked in the 

 outer one, is restricted to the plantar surface, and the front of the 

 joint is convex transversely and in the direction of the long axi& 



