 While nearly all nautiloids show gently curving sutures, the ammonoid suture line (the intersection of the septum with the outer shell) is variably folded, forming saddles ("peaks" which point towards the aperture) and lobes ("valleys" which point away from the aperture).The suture line has four main regions.The external or ventral region refers to sutures along the lower (outer) edge of the shell, where the left and right suture lines meet.