An order of sharks distinguished from all other sharks by the following combination of characters: trunk cylindrical; head conical to cylindrical, not laterally expanded; two dorsal fins, lacking spines; five gill slits, all anterior to the pectoral fin base or the last two above the pectoral fin base; mouth large, its gape extending well beyond the eyes; spiracles usually present, small and well behind the eyes; eyes without a nictitating membrane; barbels absent; and ring-type intestinal valve. Lamniform sharks are ovoviviparous. Fertilized eggs are enclosed in egg cases in the oviducts (uteri), where they undergo embryonic development, ordinarily from the energy supplied by the yolk. Although the reproductive event may begin with each uterus containing about 20 fertilized eggs, normally only one or two in each uterus reach full term. The gross loss of potential progeny is explained by oophagy or uterine cannibalism, an uncommon phenomenon in which the largest (and presumably oldest) fetuses are free in the uterus to feed on the remaining eggs and developing embryos, thus cannibalizing their potential siblings. This reproductive strategy sacrifices numbers for a few large, strong pups that are like small adults, ready to fend for themselves.