ARTIODACTYLA. 



579 



articulates with the trapezium, mc. 2 with the trapezium, trape- 

 zoid, and magnum, mc. 3 with the magnum and unciform, 

 mc. 4 and 5 with the unciform (Fig. 297 bis. A). If this arrange- 

 ment is maintained with reduction in the digits, the reduction is 

 spoken of as inadaptive reduction (297 bis, B) ; inadaptive 

 reduction is cliaracteristic of the ancient forms. In adaptive 

 reduction the upper articulating surfaces of mc. 3 and 4 

 broaden out and push the lateral metapodia outwards, so 



Fio. 297 bis. — Manas A of Oreodon, B of Xiphodon, showing inadaptive reduction ; C of Sus, 

 D of Tragulus, showing adaptive reduction (from Zittel). c cuneiform ; I lunar ; m mag. 

 num ; R radius ; s scaphoid ; td trapezoid ; tz trapezium ; U ulna ; u unciform ; i-v digits 

 numbered. 



that mc. 2 ceases to articulate with the magnum (Fig 297 bis, C, 

 D). The pes is always ahead of the manus in reduction and in 

 the fusion of the metapodia. It never possesses even the meta- 

 tarsal of digit 1. 



It is said that the metapodia when fused in the adult retain 

 their distinctness in the embryo, thus corresponding with the 

 older forms. But it must not be forgotten (1) that this distinct- 

 ness is confined to the stage of cartilage, (2) that the metapodia, 

 however close the coalescence may be, always show distinct 

 traces of their double origin in the adult (separate medullary 

 cavities, and grooved surface), and (3) that no traces of the 



