Rūpa-kalāpa: 'material group', material unit, designates a combination of several physical phenomena constituting a temporary unity. Thus, for instance, the so-called 'dead matter' forms the most primitive group, consisting only of 8 physical phenomena, called the 'pure 8-fold unit' or 'octad' suddhatthakakalāpa to wit: the 4 elements the solid, fluid, heat, motion; colour, smell, taste, nutriment pathavī, āpo, tejo, vāyo; vanna, gandha, rasa, ojā In Vis.M, and elsewhere, it is also called ojatthamaka-kalāpa 'the octad with nutriment as the 8th factor'.
The simplest form of living matter is the '9-fold vitality unit' or 'life-ennead' jīvita-navaka-kalāpa formed by adding 'vitality' to the octad. Seven decades, or units of ten dasaka-kalāpa are formed by adding to the 9-fold unit one of the following material phenomena: heart physical seat of mind, sex, eye, ear, nose, tongue or body. - See Vis.M XVIII, 4.
Maha Thera Nyanatiloka. Manual of Buddhist Terms and Doctrines, Buddhist Publication Society, first edition 1952.