[math-fun] Atlas of Finite Groups query

The Atlas of Finite Groups at http://brauer.maths.qmul.ac.uk/Atlas/ has been the subject of my recent (somewhat apprehensive) investigations. As expected, it proves to make few concessions to the neophyte. In particular, I would be grateful if some kind group-theorist could enlighten me as to the meaning of "19:9" and similar (modulo typo) expressions, occurring frequently therein, but nowhere defined! Fred Lunnon