This double keel is unique to the genus Lamna . The upper, wide keel is similar to that of the other mackerel sharks, but the second and smaller keel, located on the lower lobe of the tail fin itself, occurs only in the porbeagles. It is often makos that are mistaken for porbeagles, but the mako is a more slender fish, and it is a rich blue color above. The porbeagle also has a white patch at the posterior base of the dorsal fin. The teeth of the mackerel sharks are interesting for reasons other than their use as a genus determinant.
Most sharks have teeth that differ in the upper and lower jaws.
This differentiation is based on the separate functions that the jaws perform. The lower teeth are often narrower and more pointed, and they are used to pierce and hold the prey, while the broader serrated upper teeth are protruded, and the head is twisted, thus removing a bite of flesh. In other words, the sawlike edges of the upper teeth function as a knife, while the lower teeth serve to hold the prey, much in the manner of a person carving a roast with a knife and fork. In the jaws of the mako and the porbeagle, both the upper and lower teeth are narrow and smooth-edged, which would indicate that both sets are used for grasping and that the prey is swallowed whole. This is borne out by many studies of the stomach contents of mackerel sharks, which show many fish more or less undamaged, except for the stablike puncture wounds made by the teeth.