I found a recent and interesting article about the various types of ivory at the following address:
http://stoneplus.cst.cmich.edu/zoogems/ivory.html
Nothing very new in it but a good summary with references and it confirms the dotted line and the higher density of hippo ivory (and spermwhale ivory) than elephant ivory especially. It also says that Schreger lines are only found in elephant or mamooth ivory
Regards