ATAVISM. 163 
traversed by the excretory ducts of the gland, or forms 
a conduit whereby the young are enabled readily to suck 
the secretion. Mammary glands without ducts occur 
normally only in ornzthorhynchus and echidna. Milk- 
glands are a distinguishing feature of mammalia, being 
unknown below this class. 
Mammary glands are, as a rule, regularly and sym- 
metrically arranged along the ventral aspect of the trunk, 
in two rows. When numerous they extend along the 
thorax and abdomen into the inguinal region. The teats 
usually correspond to the maximum number of young at 
a birth, but to this rule there are many exceptions. The 
greatest number known is fourteen pairs. When the 
number of functional glands is reduced to two, traces of 
the suppressed mamme occur as supernumerary or 
accessory nipples, with or without rudimentary glands. 
The reduction in number may take place in the 
thoracic region, leaving the inguinal set functional as in 
the cow, mare, sheep, goat, deer, and antelope. The 
inguinal glands may abort, leaving the pectoral set 
functional as in man, dugong, sloth, manatee, and 
monkeys. In marsupials the milk-glands are inguinal 
in position, and protected by the remarkable fold of 
skin which forms the pouch or marsupium. 
The lemurs offer instructive modifications. The ring- 
tailed lemur (ZL. catfa), the black lemur (LZ. macaco), and 
the mongoose lemur (ZL. mongoz), possess one pair of 
pectoral mamme, as in man and quadrumana. Co- 
querel’s mouse lemur (Chzvogaleus coquereli), and the 
dwarf lemur (JZicrocebus smithz), possess three pair of 
mammary glands: one pair in the pectoral, another in 
