Helmut,

I'd be happy to tell you.  The egg case of a skate is a dark, leathery,
rectangular capsule anywhere from 25mm to 150mm long.  It has streamers on
eacg corner which are actually hollow and allow the embryo to pump fresh sea
water through the case in order to breath.  They are often washed ashore and
are found in the drift line with seaweed and shells.  They are called
mermaid's purses because of their shape and because they are hollow.  I will
send you an image off list.

Tom Eichhorst in New Mexico, USA