The book says:
The plural is regular. Note that the -i- of the base is lost only in the genitive singular, and in the vocative of words like filius.
What about the dative and ablative plurals with their -is- endings?
Singular
Nom - praesidium parvum
Gen - praesidi parvi
Acc - praesidium parvum
Dat - praesidio ...

News of Latin For Beginners by D'Ooge