LIMITS OF HEREDITARY CONTROL 863 
and from which fine hairs extend. The plate also has two other 
marginal hair pits containing hairs, one situated well toward 
each corner of the base; but these emerge from beneath the scute, 
and consequently the margin of the latter shows no corresponding 
emargination, as in the case of the more centrally located pits. 
The secondary scutes are also more or less wedge-shaped, but, 
unlike the primary ones, have their bases directed toward the 
anterior margin of the band (fig. 20). In brief, their bases form 
the anterior limit of the exposed part of the band. The apex of 
the wedge is blunt and forms a small part of the posterior margin 
of the band. Through the median axis of the scute a faint groove 
extends from the middle of its base to a point near the apex. 
Upon the removal of the scute it is found that the groove is due to 
the depression of the suture between the two adjacent bony 
plates, directly above which the scute is situated. The secondary 
scutes are clearly composite structures; that is, they have been 
formed during the process of their evolution by the union of 
some three or four elemental units of regions such as are seen in 
the scapular and pelvic shields, which exhibit conditions more 
primitive than that of the bands. 
In addition to the four hair-pits already alluded to, several 
others belonging to the associated hair group appear on the upper 
surface of the plate. The most prominent of these is a row of 
six or seven extending along each side of the primary scute mark- 
ing. These pits possess very fine hairs which often extend above 
the surface of the band, especially in young animals. There are 
also faint indications of other hair-pits, both on the convex area 
of the plate as well as on its anterior unexposed third, but it 
is entirely beyond the scope of this paper to enter into a detailed 
description of them. It is sufficient here merely to note that each 
plate corresponds to a rather distinct hair area of other mammals, 
and consequently has a definite number of hairs included within 
its limits. ; 
It will be evident from the foregoing account that for each 
primary scute there is a corresponding plate in which is imbedded 
a definite group of hairs. A count of the primary scutes will 
therefore also give a count of the plates and of the hair groups. 
