B. baini，化石發現於南非卡魯盆地的貘頭獸集合地帶，是緩龍的模式種。頰部骨頭中度發展。口鼻部寬廣、圓形，上下頜各15到16對重疊的牙齒。此種被認為是早期的鋸齒龍類，缺乏明顯的自有衍徵。
B. seeleyi，化石也是發現於南非卡魯盆地的貘頭獸集合地帶，是較不常見的物種。Boonstra認為此種是個有效種，但Lee認為此種是更先進鋸齒龍類的近親。此種似乎是Nochelesaurus與Embrithosaurus的近親。與體型相近、數量更多的B. baini相比，此種的頰骨更大。上下頜各19到20對明顯重疊的牙齒。
參考資料
Boonstra, L. D. 1969, "The Fauna of the Tapinoephalus Zone (Beaufort Beds of the Karoo)," Ann. S. Afr. Mus. 56 (1) 1-73, pp. 29-32
Edwin H. Colbert, 1965, The Age of Reptiles, The World Naturalist, Weidenfeld and Nicolson, London, pp.52-3
Barry Cox, R.J.G.Savage, Brian Gardiner, Dougal Dixon, 1988 Illustrated Encyclopaedia of Dinosaurs and Prehistoric Animals
Carol Lane Fenton and Mildred Adams Fenton, 1958, The Fossil Book, Doubleday & Co., Garden City, New York, p.306
Kuhn, O, 1969, Cotylosauria, part 6 of Handbuch der Palaoherpetologie (Encyclopedia of Palaeoherpetology), Gustav Fischer Verlag, Stuttgart & Portland,
Lee, MSY (1997), Pareiasaur phylogeny and the origin of turtles. Zool. J. Linnean Soc., 120: 197-280
