ITS ANCESTOKS AND KELATIONS. 25 



column is said to consist of seven cervical, fourteen 

 or fifteen thoracic, six or seven lumbar, and three to 

 five sacral vertebras. The tail is long and tapering, 

 much longer than in any existing ungulate, as it 

 must have reached quite to the ground in walking. 

 The scapula or shoulder-blade has a very oval form, 

 resembling that of a carnivore more than that of any 

 existing ungulate. The clavicles or collar-bones, as 

 previously mentioned, are lost* 



* In using this expression the assumption is made that 

 Phenacodus, and, in fact, all other mammals, are derived 

 from forms having clavicles, and that the absence of these 

 bones is a case of specialization, an assumption supported by 

 the facts that the presence of clavicles is the rule in birds, 

 reptiles, and amphibia ; that they are well developed in vari- 

 ous orders of mammals not otherwise closely associated, as 

 marsupials, edentates, insectivores, and primates; and that 

 they are also found, though often in a more or less vestigial 

 condition, in rodents and carnivores. These facts all tend to 

 show, if they do not conclusively prove, that the presence of 

 the clavicle is the typical condition, notwithstanding its com- 

 plete absence in extensive groups of mammals, as the ungu- 

 lates and cetacea. 



Since the above was in type the discovery has been an- 

 nounced of the presence of a rudimentary and transient clav- 

 icle in an early embryo of a sheep. This affords a complete 

 confirmation of the view above expressed, and is a most as- 

 tonishing instance of the persistence of a structure in the 

 embryonic condition, which has, as far as our evidence tells, 

 been absent in the adults during the whole of the Tertiary 

 period. H. Wincza, "Ueber ein transitorisches Eudiment 

 einer knochernen Clavicula bei Embryonen eines Ungulaten." 

 Morphol. Jahrbuch, xvi. p. 647. 1890. 



