The Middle Silurian Trimeroceratidae resembled the related Hemiphragmoceratidae in having the shell aperture visored by encroaching lappets. The two families mainly differed in the structure of the siphuncle, that of Trimeroceratidae being slender and without actinosiphonate deposits.
Characters (from Sweet 1964): Exogastric, mostly compressed brevicones with slender, empty siphuncles. Aperture visored; mature peristome with median dorsal sinus.
[T64] Teichert, C. 1964. Morphology of hard parts. In Treatise on Invertebrate Paleontology pt. K. Mollusca 3. Cephalopoda – General Features – Endoceratoidea – Actinoceratoidea – Nautiloidea – Bactritoidea (R. C. Moore, ed.) pp. K13-K53. The Geological Society of America and the University of Kansas Press.
Christopher 00:23, 16 January 2011 (UTC)