A cross wall or partition.

1. A morphological term used particularly with reference to the Cephalopoda, whose shells are divided internally into a series of chambers (camerae) by septa which are generally concave towards the anterior. There is an opening (foramen) in each septum and this is usually bordered by a collar (septal neck). Septal necks are forward-pointing (prochoanitic) in ammonoids (Ammonoidea) and backward-pointing (retrochoanitic) in nautiloids (Nautiloidea). In many ammonoids the septum becomes fluted and there has been much discussion concerning the function of the complex fluting.

2. The radially arranged plates that occur in the corallum of corals. The first-formed septa are called ‘prosepta’ and are usually larger than the metasepta, which occur between them.