Evolution of Animals 483 



of the articular line of the humeri, then 3 to 5 cervicals can 

 be distinguished. In mammals there are rarely 6, usually 7, 

 seldom 8 or 9. But even this degree of variability renders 

 permissible the view that land forms may have appeared be- 

 tween the carboniferous and recent epoch, in which the usual 

 number of seven became fairly steadily established ; this resulted 

 from two changes, the absorption of the anterior vertebral 

 ribs, and the shifting backward of the pectoral girdle and line 

 of insertion into the humeri. But the shifting backward would 

 be a morphological one, rather than in actual distance relation. 

 For, while the ribbed cervical vertebrse in front of a line joining 

 the humeri in Cryptohranchus, Chondrotus, and others are as 

 long as the dorsals, in mammals they are nearly always much 

 shorter. A suite of genera of amphibio-mammals from the 

 close of the permian or early part of the triassic age is here 

 greatly to be desired, as we feel fairly confident of their former 

 existence. Some of the recently described Aistopoda from 

 Ireland and this country seem to be the desiderated types. 



In the relation of the skull to the lower jaw, and in the struc- 

 ture of the latter, the urodele Batrachia approach much more 

 nearly to manunals than do the reptiles. For in the former 

 the squamosal closely covers the quadrate, and lies directly 

 above and outside of the articular surface. The quadrate 

 however still acts as the articulating bone for the lower jaw. 

 Now according to one view the quadrate and articular in 

 mammals slip inward to form the malleus and incus bones 

 of the ear, while the squamosal directly articulates then with 

 the dentary. The retention of the quadrate as a very evident 

 bone in reptiles, and its broad fusion with the articular, as 

 one of six bones in the lower jaw, form a grave objection to 

 the reptilian view of mammalian ancestry. 



Lull {159: 1) has pointed out that "the laws which govern 

 digital reduction among vertebrates lead to an interesting 

 grouping of the Mammalia with the Amphibia, in which the 

 order of reduction is first digit 1, then digit 5, as contrasted 

 with Sauropsida — reptiles and birds-^ -in which the fifth digit 

 is invariably the first to disappear, folloived by digit 1." 



