THE INTERMEDIUM IN BIRDS. 7 



In young birds of certain species it is seen as an ascending spur occupying the fossa on 

 the lower anterior face of the tibia. In mature birds, as far as I have discovered, it 

 becomes absorbed. 



The wide, oblique, tendon -like bridge, which spans the fossa in the heron and many 

 other birds, has no relation with the intermedium. 



In Laelaps and some other Dinosaurian Reptiles, this bone is seen as an ascending spur 

 of the coossified tibiale and fibulare. In Ornithotarsus, as figured by Professor Cope, an 

 appearance of the tarsus is represented, which is not unlike that shown in the early 

 stages of the Tern. See plate, fig. 1. 



It is possible that the intermedium in Ornithotarsus was a separate bone, and that it has 

 been lost in the fossil, as a distinct grove or fossa is seen on the lower anterior face of 

 the tibia, while no corresponding spur is seen on the tibiale, a slight elevation only being 

 discernible. 



Professor Marsh (10), in a paper on the Limbs of Sauranodon, an animal related to 

 Ichthyosaurus, has presented some novel views in regard to the homologies of the 

 intermedium. 



cf The fee limb is there figured with the femur of the usual character. Its distal face 

 having, however, three articular surfaces, to which are applied three bones in the shape 

 of irregular formed disks ; these he interprets as tibia, intermedium and fibula. 



He suggests that the intermedium belongs to the epipodial, or second series of bones 

 represented by the radius and ulna, and tibia and fibula, and that in the process of differ- 

 entiation the intermedium has been crowded down into the mesopodial or first series of 

 tarsal bones. 



My interpretation, based on the admirable figure he presented, would be that the bone 

 which he indicates as the intermedium is really the fibula, and the bone which he repre- 

 sents as the fibula, is an outer tarsal bone, which, with its metatarsal and phalangeal 

 bone in series becomes obliterated in time; that in the process of differentiation the 

 intermedium is as likely to be partially compassed by the distal extremities of the tibia 

 and fibula, as that a third bone of this segment had been crowded down into the tarsal 

 series. However this may be, Professor Marsh has discovered a most interesting stage in 

 this highly primitive condition of the bones as shown in Sauranodon. 



In the following outlines, figures are presented of a portion of the leg of a salamander, 

 a number of embryo birds, young birds, and Dinosaurian reptiles. These are given in 

 series, so that a comparison may be made between the different stages of the inter- 

 medium in each of these forms. In the first series, figs. 1, 2, 3, 4, the intermedium 

 is seen as a separate bone. In the second series the intermedium has united with the 

 other tarsal bones and assumes the appearance which has been described as the ascending 

 process of the astragalus. In the third series a similar condition is seen. 



For explanation of lettering, see explanation of plate. The lettering being the same 

 with the exception that tf indicates the tibiale and fibulare connate. 



