Full Idea
Specific essence differs from generic essence by being demarcated: individuals are demarcated within species by dimensionally defined material, but species within genus by a defining differentiation taken from the form.
Gist of Idea
Specific individual essence is defined by material, and generic essence is defined by form
Source
Thomas Aquinas (De Ente et Essentia (Being and Essence) [1267], p.95)
Book Reference
Aquinas,Thomas: 'Selected Philosophical Writings', ed/tr. McDermott,Timothy [OUP 1993], p.95
A Reaction
It clearly won't be enough to define an individual just to define its material and its shape. The material might also be essential to the genus, as when defining fire. Probably not very helpful.