15 NEW YORK STATE MUSEUM 



hedral zone. Excellent reflections were, however, obtained from 

 all of these scalenohedrons and the closeness in agreement of the. 

 observed angles with theoretical values, taken, together with the 

 fact that the rhombohedrons M and t, whjich truncate the polar 

 edges of U and V respectively, lie well in the zones of these faces,, 

 establish their identity beyond peradventure. The negative scalen- 

 ohedron TV (4.16.20.3) which is invariably present is characterized 

 by small, sharp and brilliant faces. 



Twinning. A very marked tendency toward twinning parallel 

 to the plane e (01 12) results in the production of thin flat exten- 

 sions of one individual of the pair and the formation of a deep 

 reentering angle as shown in figure 32. 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 member of the twin above noted is, however, 

 unique and seems to indicate a metagenic rather than a paragenic 

 mode of twinning. 



The calcite crystals found in the fossil remains ofRhynchonella 

 wilsoni show combinations of the supplementary type illustrated 

 in figures 34 and 35. The crystals though small are remarkably 

 brilliant and give excellent reflections in all zones. Of the ob- 

 served forms M (4041), e (0112), r (1011) and v (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 q of 

 the principal type. This form appears as a series of well developed 

 planes having none of the vicinal characters which mark the de- 

 velopment of q of the principal type. The scalenohedron- A (3142) 

 occurs as a series of narrow faces between v and r. Traces of the 

 characteristic twinning which mark the crystals of the principal 

 type are here noted; the twinning tendency is, however, very weak 

 and only finds expression in an occasional shallow reentering 

 "gash." 



