Detlef, thank you for the picture. I'm duly aware that the interstitial zone of hippo tusks sometimes produce dark inclusions along the lateral margins, but similar "spots in a row" can be found in other types of ivory: The inner cementum of walrus or longitudinally along deep age cracks in tusks of the proboscids, for example. I was just hoping that maybe you had an academic reference up your sleeve to tie them to the hippo.
In general I think one should exercise care in assigning a source species to any ivory based on one character alone. -this goes x10 when we're doing it from photographs of course.
I can recommend the article
"Unusual appearance of Schreger-like pattern in Hippopotamus amphibius ivory" (Simms, 2010). -I don't agree with their conclusions, but their findings illustrate this point very well.
Best wishes, - Thor