Scapanorhynchus sp. Shark Teeth

Looking at this array, I despair of ever being able to sort my collection! How were you able to id these all as Scapanothynchus sp? Is there a good source for a taxonomy key for these? 

@CAT B, I usually reference "The Collector's Guide to Fossil Sharks and Rays From the Cretaceous of Texas" by Welton and Farish. I got my copy from DPS (https://www.dallaspaleo.org/Store). Otherwise, there's a wealth of teeth in others' collections here to compare to and you can always make a post on fossil ID for help.