The ontogeny, palaeobiology and systematic palaeontology of some lower liassic Belemnitida
The Belemnite Marls of the Dorset coast constitute the jamesoni and ibex Zones of that area. This study is essentially a revision of the total belemnite fauna of these beds, with a preliminary appraisal of the belemnites from the overlying basal Green Ammonite Beds (davoei Zone). Twelve morphospecies are described which are defined by their rostral ontogeny and other morphometric data. Probable sexual dimorphism into relatively stout and slender rostra is demonstrated for Four species (Belemnites longissimus Miller, B. charmouthensis Mayer, B. imus (Lang) and B. cricki (Lissajous)). It is recognised that, at different localities and horizons, single species may be represented by assemblages that vary with respect to age and sex of individuals, and it is concluded that such belemnite assemblages are at least partly due to local population structures analagous to those that are found in modern cephalopods. Early ontogeny, rostral form and function, epirostra and surface ornamentation are briefly discussed with reference to the species described.