352 On the Evolution of the Vertebrata, etc. [April, 
perfect mechanical arrangement, the change is clearly an advance. 
The same progressive improvement is seen in the development of 
distinct facets in the cubito-carpal articulation, and of a tongue 
and groove (“intertrochlear crest”) in the elbow-joint. In the 
vertebre the development of the interlocking zygapophysial artic- 
ulations is a clear advance. 
Progress is generally noticeable in the dental structures ; unlike 
the marsupial line the earliest dentitions are the most simple, and 
the later the more complex. Some of the types retain the primi- 
tive tritubercular molars, as the Centetidze, shrews and some 
lemurs, and many Carnivora, but the quadritubercular and its 
derivative forms is by far the most common type in the recent 
fauna. The forms that produced the complicated modifications 
in the Proboscidia and Diplarthra appeared latest in time, and the 
most complex genera, Bos and Equus, the latest of all. The 
extreme sectorial modifications of the tritubercular type, as seen 
in the Hyznidz and the Felidz, are the latest of their line also. 
Some cases of degeneracy are, however, apparent in the mono- 
delphous Mammalia. The loss of pelvis and posterior limbs in 
the two mutilate orders is clearly a degenerate character, since 
there can be no doubt but that they have descended from forms 
with those parts of the skeleton present. The reduction of flex- 
ibility seen in the limbs of the Sirenia and the loss of this char- 
acter in the fore limbs of the Cetacea are features of degeneracy 
for the same reason. The teeth in both orders have undergone 
degenerate evolution, to extinction in the later and existing forms 
of the Cetacea. The Edentata appears to have undergone de- 
generation. This is chiefly apparent in the teeth which are 
deprived of enamel, and which are wanting from the premaxillary 
bone. A suborder of the Bunotheria, the Tzniodonta of the 
Lower Eocene period, display a great reduction of enamel on the 
molar teeth, so that in much worn examples it appears to be 
wanting. Its place is taken by an extensive coat of cementum, 
as is seen in Edentata, and the teeth are ever rootless as in that 
order. It is probable that the Edentata are the descendants of the 
Tzniodonta by a process of degeneracy. 
Local or sporadic cases of degenerate loss of parts are seen in 
various parts of the mammalian series, such are toothless Mam- 
malia wherever they occur. Such are cases where the teeth be- 
come extremely simple, as in the honey-eating marsupial Tarsipes, 
