Sunday sea urchin
Interestingly, uni jaw structures were used to determine the longevity of the red uni, with surprising results. "The largest reported red sea urchins, with body diameters over 19cm, are from British Columbia, Canada, (Bureau, 1996) and with estimated jaw lengths of about 2.8 cm would be expected to be around 200 years old (Fig. 2A)." 
I'd have never suspected any of this about these small round animals with a water-vascular system. And yet these are just a few unique traits among many that uni possess. Below is a photo of an uni's jaws and teeth, you may be able to identify some of the features.
Edit: cross posted to invertebrates (view entry).