The ontogenies of trilobites and their significance

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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.

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