Siphuncular structure in a Silurian narthecoceratid nautiloid cephalopod from the Island of Gotland

The material of Donacoceras gotlandense n.sp. (Orthocerida, Fam. Narthecoceratidae), embedded in the Llandoverian Lower Visby mudstone on the Island of Gotland, Sweden, consists mainly of isolated siphuncles. The connecting ring is thick and fully calcified. It is traversed by numerous pores, resembling in this respect that of actinocerids. This type of connecting ring is here termed calcified-perforate. The similarity in the structure of the connecting rings indicates a closer relationship between actinocerids and orthocerids than was previously envisaged. The endosiphuncular deposit is of a modified conical type. Because the deposit is thin dorsally but thick ventrally, the endosiphuncular canal is situated close to the dorsal side. The deposit shows extensive variations in structure and shape. It is often composed of radially arranged calcareous lamellae in which crystallites have a feather-like aspect. In its adoral (towards the aperture) growth region the deposit increases gradually or abruptly in thickness. The growth lines in the deposit are, therefore, in varying degrees oblique to the long axis of the siphuncle or, in some places, practically perpendicular to this axis.