South-eastern Australia is one of the wettest regions of a predominantly dry continent. From the rainforests of the Victorian Alps to coastal wetlands, Victoria is endowed with a great diversity of freshwater environments.
On the southern side of the Great Dividing Range, streams merge with rivers and estuaries before flowing on to the coast. On the range's northern side, melted snow eventually runs into the Murray River and its tributaries, then along an ancient system of meanders and billabongs through the arid mallee to Lake Alexandrina and the Murray mouth in South Australia.
Frederick McCoy noted that several species from catchments draining into the Murray did not occur in those draining into the southeast. In his comments on the Golden Perch for example he noted that the fish was 'Common in the River Murray and its branches, but not found in any river of Victoria flowing southwards to the sea'.
McCoy was also interested in the variety of freshwater crayfish found in different drainage systems across Victoria. The Prodromus includes just two of these varieties: the Murray Spiny Crayfish and the Yarra Spiny Crayfish. However unpublished plates reveal two further sets of illustrations, indicating that he was trying to confirm whether these examples were variants of the Murray Spiny Crayfish or distinct species in their own right, identified with a particular catchment.
Several other Victorian freshwater fish were also drawn but never published. Together with images of crustaceans and reptiles, they form a vivid series of Victoria freshwater fauna.