The teeth of several species of sharks can only be told apart by
their striations. If you've never seen the real thing, you may
easily mistake fine cracks for striations.
The picture on the left shows a scan of the crown of a Sand tiger tooth. The striations are clearly visible as ridges on the lower 3/4 of the crown. The tip of the crown is smooth. Striated sand tigers are quite rare at Bownie's, and the best example I could find is this damaged 11mm long tooth.
The picture on the right is a scan of one of these rare, pitch black teeth you can find at Brownie's. I suspect these are not Miocene teeth, but from an older period. There are several eocene exposures along the bay and its tributaries. It is very well possible that this tooth has washed in from elsewhere recently, or that it washed in 15-20 million years ago and got mixed in with the fresh teeth then.
Anyway, this 24.5mm long tooth shows another example of strong striations.
Note that the two pictures are not on the same scale!