The skull of the platypus and echidnas are highly specialized in a manner divergent from those of all other groups of mammals, fossil or living.
Echidnas come as two species in two separate genera and with quite different habits.
Echidnas are a successful and at least modestly varied group. Frequently, a suite of characters undergoes a complete transformation before other characters change at all. The concept of mosaic evolution dictates that organs will evolve in different ways to meet varying selective pressures. We would not deny Cuvier's inference today, but only his initial premise of tight and ubiquitous correlation. Evolution is mosaic in character, proceeding at different rates in different structures.