New York Consolidated Laws, Estates, Powers and Trusts Law - EPT § 1-2.14 Per stirpes

A per stirpes disposition or distribution of property is made to persons who take as issue of a deceased ancestor in the following manner:

The property so passing is divided into as many equal shares as there are (i) surviving issue in the generation nearest to the deceased ancestor which contains one or more surviving issue and (ii) deceased issue in the same generation who left surviving issue, if any.  Each surviving member in such nearest generation is allocated one share.  The share of a deceased issue in such nearest generation who left surviving issue shall be distributed in the same manner to such issue.