CALCITES OF NEW YORK I13 
nohedron, having the indexes (9.4.13.2) was noted in most of the crystals 
measured. This scalenohedron, to which the letter has been assigned 
is present in best development on the combination shown in figure 2. A 
more complex combination which is shown in figure 3 yielded in addition 
to the above forms the positive scalenohedron 8: (8.4.12.1) in the zone 
[4041.1120], present as a series of bright well developed faces, and the posi- 
tive scalenohedron T (4261). The latter form which was noted on but 
one crystal was mainly identified by zonal relations. On one crystal 
presenting the combination shown in figure 4 two faces of the negative 
scalenohedron p: (1341) were noted; the form, however, was not repeated 
in any of the crystals subsequently measured. A negative scalenohedron 
having indexes closely approaching (6.21.27.5) was found repeatedly 
throughout the type. The planes of this form although bright are somewhat 
rounded giving poor reflections of the goniometer signal, and no consistent 
readings could be obtained. The form must therefore be classed as doubtful. 
A very marked tendency toward twinning parallel to the plane 6. (0112) 
results in the production of thin flat extensions of one individual of the pair 
and the formation of a deep reentering angle as shown in figure 5. So 
common is this form of twinning that it is rarely absent from crystals of 
this occurrence to which it gives a distinct character. Twinning according 
to this law is common in calcite crystals and examples of it may be found in 
almost every important occurrence. The abnormal extension of one mem- 
ber of the twin above noted is, however, unique and seems to indicate a 
metagenic rather than a paragenic mode of twinning. 
Type III [pl. 21, fig. 6.] Small calcite crystals of this type occur 
lining the fossil remains of Rh ynchonella wilsoni which thickly 
stud portions of the Helderbergian limestone, overlying the Rondout. 
The crystals though small are remarkably brilliant and give excellent 
reflections in all zones. Of the observed forms, m. (4041), 6. (0112), 
p- (1011) and K:(2131) are common to the crystals previously described 
from the underlying beds of the Rondout limestone. The scalenohedrons 
are all of the zone [0112.1011]. The scalenohedron e: (4156) here replaces 
