Origin of Mammalian Carpus and Tarsus. 93 



two elements. There are probably five tarsalia and a distinct 

 centrale. 



In Procolophon (7) the tarsus consists of six bones, but the inter- 

 medium is manifestly united with the tibiale, and it is not improbable 

 that the centrale is united with the fibulare. There are only four 

 tarsalia ossified. In other primitive Diaptosaurians five tarsalia are 

 ossified. 



In Sphenodon (14) ■ the structure and development of the tarsus 

 have been very fully worked out by Howes and Swinnerton ; and 

 though Sphenodon has advanced far from the point of separation of 

 the Diapsida and the Synapsida, it gives us perhaps a better idea of 

 the primitive type of the Eeptilian tarsus than we get from most of 

 the fossil specimens. Here we find that the tarsus in its early 

 development consists of a moderate-sized fibulare and tibiale, a large 

 intermedium, a rather small centrale, and four tarsalia. 



Unfortunately no Therocephalian tarsus is known, so that the 

 intermediate stages between the Cotylosaurian types and the 

 Dicynodont are at present hypothetical. 



The Dicynodont type is fortunately now fully known. A few 

 years ago I described the tarsus of Oudenodon (12), and since then 

 have had the opportunity of examining two other Dicynodont tarsi. 

 The tarsus of Oudenodon trigoniceps has been somewhat more fully 

 developed, and a slight modification is required of my previous 

 description. The small element which I regarded as the centrale 

 has been found to be really a part of the tibial. The intermedium, 

 however, seems to be a distinct element. The first row of the tarsus 

 thus consists of a large semicircular tibiale, and a somewhat larger 

 fibulare, with a small intermedium fitted in between them. The 

 distal row is formed of four bones, of which the first is large and 

 almost like a metatarsal. Between the tibiale and the first tarsale 

 there is a moderate-sized gap in which it is moderately certain there 

 was a cartilaginous centrale. The second Dicynodont tarsus I have 

 examined probably also belongs to a species of Oudenodon, and while 

 it agrees closely with that of 0. trigoniceps it shows a distinct ossified 

 centrale between the tibiale and first tarsale. The centrale is not 

 fully ossified like the other elements, but has the appearance of an 

 imperfectly ossified cartilaginous element. The third tarsus is a 

 specimen in the Albany Museum — probably belonging to a species 

 of Dicynodon. Here the most noteworthy feature is the presence of 

 a large fully ossified centrale, which articulates with the tibiale and 

 with probably all four tarsalia. The Dicynodont tarsus, with the 

 exception of having a distinct intermedium, is thus seen to be 

 practically of the mammalian type. 



