Without attempting to answer this question categorically, it may be pointed out that within the limits of the family (Ptychoderidae) which is especially characterized by their presence there are some species in Y art dY YY cts, posterior limit of collar.
Bay ll c t ' LY Q " yy be Pony.
It's a YY MM format (for example, 06.12 is December 2006).
The equations to the tangent and normal at the point x' y are yy' = 2a(x+x) and aa(y - y')+y'(x - x')=o, and may be obtained by general methods (see Geometry, Analytical, and Infinitesimal Calculus).
