this fine web site which not only has a great picture of a porpoise skull but also shows the teeth of porpoise and dolphin side by side. My conclusion on the dental dilema is that the only reason for the different shapes is so that taxonomists can tell dolphins from porpoises....
The skull is weird. Apart from the curious concave shape at the front, there seem to be no eye sockets. There are a couple of decent photos of a live porpoise here. The eyes are small and very much to the side and low down. The other key feature is the bulbous forehead - quite the opposite of the skull shape and you'd be forgiven for thinking that the skull must be from a different animal altogether.
The answer seems to be connected with the fact that these animals use echolocation rather than sight to find their way about and to hunt prey. They produce the high pitched ulrasounds in organs in the nasal passages of the head and these then pass into an oil filled organ called the 'melon' which aids the production and focussing of the ultrasound beam. The melon sits in the concave front of the skull and creates the convex profile of the head.
Fascinating animals porpoises.