The ontogenies of trilobites and their significance

Abstract

The paper summarizes an earlier one on "The Development of Leptoplastus salteri (Callaway) and other Trilobites," published in 1925, and discusses the conclusions, mainly phylogenet-ic, that immediately arise. After a bibliography, the parts of the trilobite cephalon are defined: primitive sutures subdivide it into morphological units[long dash]cranidium, pareiae (free cheeks), epistome and hypos-tome (labrum). An extended discussion of trilobite ontogeny is given under the following heads: natural divisions in the ontogeny; significance of the protaspis, and of the later stages of development; the stages of the development of Leptoplastus salteri* and their significance; limitations to the phylogenetic significance of the stages of development; significance of the ontogenies for relationships between the families of trilobites, and between trilobites and other arthropods; and final considerations. An appendix gives a tabular statement of the chief features in the ontogeny of Leptoplastus and the corresponding conclusions drawn from them.