6 EDWARD S. MORSE ON 



intermedium has been crowded out and up, so that its distal end is on a line with 

 the proximal margin of the tibiale and fibulare. The centrale has increased in size 

 so that its transverse diameter is equal to that of the three metatarsals, to which it finally 

 becomes attached at a later stage. 



In the Petrel, Procellaria pelagica, fig. 5, a stage is represented, similar to that 

 of the Tern, fig. 2. A new tai'sal bone appears (indicated on the plate with a 

 question mark), corresponding to the rudimentary first toe. This bone was clearly defined 

 in both legs. In appearance it looks like the proximal end of the first metatarsal bone, 

 with its corresponding tarsal. In only one other bird has a bone at all resembling 

 this been seen, and that is shown in the figure of the Eider Duck (fig. 8, indicated 

 by a query mark). In this case it appears like a second tarsale. In both cases this 

 bone appears on the tibial side of the leg. It will be unsafe to hazard a conjecture 

 as to what this bone represents, without further examination of other species. 



In the Sea Pigeon, Uria grylle, the intermedium is a prominent, wedge-shaped bone, 

 and quite separate from the other tarsal bones, though the embryo was far advanced. 

 Its large size is interesting in connection with the fact that the fore limb shows 

 conspicuous rudimentary nails on the secondhand third fingers. (See figs. 11, 12.) 



In fig. 8, the tarsus and a portion of the other bones of the leg of the Eider Duck, 

 Somateria mollissima, are represented. In this embryo the intermedium was clearly seen 

 between the tibia and fibula, the tibiale and fibulare being widely separated. 



In the Herring Gull, Larus argentatus, the intermedium is not large, the near tarsal 

 bones are about uniting, yet the intermedium is still free, and somewhat removed from 

 them. 



In the Southern Black backed-Gull, Larus dominicanics, the intermedium is in nearly 

 the same condition. In the Great Auk, Aptenodytes Pennanti, the appeai-ance of the 

 very short metatarsal bones, and the short square phalangeal bones, is extremely inter- 

 esting. The embryo was far advanced, as indicated by the long and prominent claws 

 tipping the toes. Yet the bones composing the foot are very rudimentary. 



The embryo of this species, and also that of the Southern Black-backed Gull, had been 

 preserved in strong alcohol for several years, and the cartilaginous portion of the tarsus 

 was so opaque that nothing definite could be made out in regard to the other tarsal bones. 

 The intermedium, however, in both species, was very distinct. 



From these observations it is seen that the intermedium is present in embrj^o birds as a 

 distinct tarsal bone ; that at first it is in line with the near tarsal series, that is to say 

 with the tibiale and fibulare, and also between these two bones, and consequently 

 between the distal extremities of the distal ends of the tibia and fibula. As the tibiale 

 and fibulare coalesce, the intermedium is crowded outward and upward, the tibia widening 

 at the same time to an extent equal to the transverse diameter of the near tarsal series ; 

 the intermedium occupies a position in front of the tibia, and fills a groove which is seen 

 on the anterior face of the til)ia. It is seen, furthermore, that the intermedium is the 

 last bone to unite with the coiissified tibiale and fibulare. 



The intermedium varies greatly in size in the embryos of different species. It will 

 probably be found of more common occurrence among the lower birds. 



