The present studies on trilobite morphology succeed a first part dealing with the thoracic appendages and their phylogenetic signiflcance. In preparing a second part on the cephalic appendages and the afflnities of trilobites, it became obvious that these subjects could not be satisfactorily treated without a detailed study of the larval development. Consequently I have carried out an investigation of the literature available, and have reexamined important ontogenetic series preserved in the Paleontological Museum of the University in Oslo. The studies have suggested new conceptions of the structure and origin of some of the more important trilobite characters. For this reason the present research on the larva! development is published before the description of the cephalic appendages and the general discussion of the trilobite afflnities.

The ontogenetic studies have indicated a new division of the earliest larval period. In spile of considerable difference in the structure of the protaspis, it seems possible to homologize the larvalkcharacters in the Olenellidae ( = Mesonacidae) with those in the typical opisthoparian and proparian trilobites. The protaspis of the Olenellidae has a well-developed preantennal segment, the dorsal development of which is considerably delayed in the ontogeny of the members of the other groups. The primitive segmentation of the protaspis is discussed and the existence of merely primary somites in the cephalon is emphasized. The oblique direction of the cephalic segments exposed in the primitive larvæ have led to the assumption that the transverse suture between the cephalon and the transitory pygidium, and later also between the thoracic segments, are secondary joints or hinge-lines crossing the primary segmentary borders indicated in the pleural furrows. The ontogeny also suggests a new interpretation of the cephalic sutures which have been of primary importance to the current classiflcation of trilobites. It seems possible to derive the facial suture of the Proparia and Opisthoparia from the primary marginal suture apparently present in all protaspis, and maintained in the adults of the Olenellidae and probably the Hypoparia. The Olenellidae form apparently a primitive, more or less ancestral group of trilobites which might be distinguished as a special Order for which Swinnerton's name Protoparia is suggested. The differentiation of the Class Trilobita into several Orders, according to the Salter-Beecher system, seems largely to be due to the arrested development of the preantennal segment, a phenomenon here jnterpreted as due to partial neoteny or merostasis. The arrested development seems at the same time to illustrate another evolutionary principle according to which new morphological characters first appear in the earl y ontogenetic stages .