BILL AND HAIRS OF ORNITHORHYNCHUS PARADOXUS. 153 
ancestral features of the Class. Although the hairs of Ornitho- 
rhynchus will be shown below to present many peculiarities 
which are, as I believe, ancestral, the sweat-glands are essen- 
tially similar to those of Mammalia generally. 
In the bill, the deeply placed coiled gland-tube is succeeded 
by a coiled duct which, as in many other mammals, enters the 
base of an epidermic downgrowth—the interpapillary process 
(fig. 8). The epidermic process itself, however, is by no means 
typical, but presents many special peculiarities, some of which 
support the conclusion that it is a modified hair,—sharply cut 
off above at the level of the uppermost epidermic layer, 
shortened below by retraction of the hair-bulb, so that the 
latter descends but a short distance beneath the lowest layer of 
the epidermis (figs. 8, 13—16). Nevertheless, in the young 
Ornithorhynchus the bulb-like part of the structure extends 
to a somewhat deeper level (compare fig. 16). These hair-like 
structures were briefly described in 1884 (1. c., p. 16, where, 
however, in line 11 from bottom, the words “hair papilla” 
are obviously intended for “ hair-bulb’’). 
The epidermic processes are, like the push-rods, surrounded 
below by a continuous dermal sheath, the upper edge of which 
gives rise to several papillary upgrowths (figs. 8, 13—15 
for longitudinal, figs. 9—12 for transverse, section). The 
process is continued upwards through the stratum corneum 
as a cylinder—either straight or with §-like curves,—which 
remains perfectly distinct from the epidermis around, being 
separated by a downgrowth of cells so marked that their 
direction becomes vertical. In this respect the structures 
in question resemble the push-rods (compare fig. 1 with 
fig. 8). At its upper free end this cylinder is sharply trun- 
cated so as to be flush with the surface of the bill. But in 
favorable examples it is surrounded by a distinct circular 
depression, and it may even project a little above the general 
surface (figs. 8, 18, and 15). At the posterior part of the 
upper bill the upper ends of the cylinders are remarkably ex- 
panded, so that their outline becomes funnel-shaped (fig. 14). 
The §-like curves into which many of the cylinders are thrown 
