Dogfish, skates, sharks and rays lay these strange eggs for a good reason , in these large species the embryo develop to a large size inside of the female fish , she then lay them and they hang to the ocean floor for protection, this eggs shell is pretty hard as well.
You can view some of these eggs at this site
http://images.google.com/images?sourceid=navclient&ie=UTF-8&rls=GGLD,GGLD:2005-03,GGLD:en&q=Mermaid's%20purse&oe=UTF-8&sa=N&tab=wi
and fir nore info go to :
http://www.findarticles.com/p/articles/mi_m1134/is_6_108/ai_55127878
