Cladoselache.

Cladoselache (CLAD-oh-SELL-uh-key and CLADE-oh-SELL-uh-key) is a genus of late Devonian shark which is known from extremely well-preserved fossils from the Cleveland Shale in Ohio. Some of the specimens even have gut contents. They most commonly have small, ray-finned bony fish in their guts, but some of them have shrimp-like creatures, conodonts, and one even has another shark in its stomach.

One of this shark's most notable features is its lack of scales. It only had scales on particular places, such as around the eyes and mouth. Its teeth do not seem to be suited for ripping and tearing up prey, so they seem more likely to have grasped prey and allowed the shark to swallow it whole, tail first. The presence of gut contents with the posterior of the prey facing Cladoselache's posterior, (instead of facing just any old way, as in most sharks) suggests that Cladoselache swallowed its prey tail first.

Cladoselache also had enormous pectoral fins, which it probably used to glide through the water as it searched for food or pursued its prey. It had a short, blade-like spine in front of its dorsal fin, which was not as advanced as the spike-like spines of Xenacanths, Hybodonts, and Ctenacanths.

Cladoselache was about six feet long, and was probably prey for giant placoderms such as Dunkleosteus.

Cladoselache had a large mouth in front of its head. More advanced forms, such as Cretoxyrhina (the Cretaceous "Ginsu shark") and the modern Great White, have a smaller mouth under the snout.

References:

http://geology.cwru.edu/~huwig/catalog/slides/1006.Ga.24.jpg

http://www.elasmo-research.org/education/evolution/ancient.htm

http://www.elasmo-research.org/education/evolution/cladoselache_feeding.htm

http://en.wikipedia.org/wiki/Cladoselache

http://dinosaurs.about.com/od/tetrapodsandamphibians/p/cladoselache.htm