THE INTERMEDIUM IN BIRDS. . 5 



finally a splint bone, with its attenuated distal extremity far removed from the tarsus ; the 

 tibia on the contrary, enlarging, so that its distal extremity equalled in transverse 

 diameter the two tarsal bones, which 'formed a cap on the end of the tibia, like an 

 epiphysis, and finally became merged with it. The intermedium, while occupying its 

 proper position between the tibiale and fibulare, and finally uniting with them, became 

 apparently displaced, so to speak, by standing in front of the tibia. 



This could be stated at the time with certainty : Namely, that the ascending pro- 

 cess of the astragalus was an independent bone, which finally united with the proxi- 

 mal series of tarsal bones, and that a similar process in the young chick and ostrich, as 

 figured by Huxley, and a similar spur or process as seen in the astragalus of Laelaps and 

 some other Dinosaurs, was to be looked upon as of the same nature. 



To prove the correctness of this interpretation as to whether this bone was the inter- 

 medium, it was necessary to examine the early embryo, and to find, if possible, the bone 

 occupying its true position in the tarsal series, and between the distal extremity of the 

 tibia and fibula. 



Believing that low aquatic birds would more readily yield these evidences, a visit was 

 made to Grand Menan, at the mouth of the Bay of Fundy, and at that place were 

 obtained the embryos of the razorbill auk, eider duck, sea pigeon, herring gull and 

 petrel. 



At Penikese Island in Buzzard's Bay, an opportunity was ofiered of examining the 

 embryos of the tern. Through the courtesy of the Smithsonian Institution, I had the 

 gratification of examining embryos of the southern black-backed gull, and the large 

 penguin collected by Dr. Kidder at Kerguelen Island, during the U. S. Transit of Venus 

 Expedition, and the results of these examinations were communicated verbally at the 

 meetings of the American Association for the Advancement of Science in 1874 (9) and 

 1875. 



With the hope of making these observations more complete, their publication was 

 withheld at the time. Finding the opportunity of studying other aquatic birds still 

 uncertain, these results, slight though they may be, are given. 



The accompanying plate presents in outline the tarsal joint of the following birds : 

 The common Tern, Sterna hlrundo ; Petrel, Procellaria pelagica ; Eider Duck, Somateria 

 mollissima ; Sea Pigeon, Uria grylle ; Common Guillemot, Lomvia troile ; Herring Gull, 

 Larus argentatus ; Eazor bill Auk, Utamania torda ; Southern black backed Gull, Larus 

 dominicanus, and Great Auk, Aptenodytes Pennanti. 



An additional figure is added of the right hand limb of the Sea-pigeon, as it shows the 

 appearance of rudimentary nails on the second and third fingers. 



In the figures of Sterna hirundo, figs. 1-4), different stages of the tarsus are represented. 

 Fig. 1 shows the intermedium distinctly wedged between the tibia and fibula, with 

 its distal end in line with the distal margins of the tibiale and fibulare. The centrale 

 is still small. In a later stage, as represented in figs. 2, 3, and 4, the tibia has widened 

 considerably, though the tibiale and fibulare have not yet united. The intermedium 

 now appears in front of the tibia, though still on a line with the tibiale. The separation 

 of the distal end of the fibula and its tarsal bone has increased. The centrale has 

 slightly increased in size. In fig. 2, the tibiale and fibulare have coalesced. The 



