Summary of Canto: Inf XVIII

1-9     Malebolge as castle: wall, moats, bridges, pit
10-18   similes: moats around castles and over-arching bridges
19-21   leftward movement after Geryon has departed
22-25   first ditch: double file of sinners 
26-33   simile: bridge in Rome over Tiber during Jubilee Year
34-39   the punishing demons with their whips
40-51   Dante recognizes Venedico Caccianemico
52-63   Venedico's pandering and that of other Bolognesi
64-66   a demon smites him and sends him off
67-74   Dante and Virgil mount to the crown of the bridge
75-78   Virgil: now Dante can watch the other group of sinners
79-99   view of the seducers: Jason
100-108 the second ditch: sound of whining, stench
109-114 to see within, they mount to the bridge's crown 
115-126 flattery: Alessio Interminei of Lucca
127-136 flattery: Thaïs the whore; abrupt ending of canto