abdominal segment had become reduced." From this fact, and the absence in Limulus of anything comparable to the central lobe of the genital operculum, the higher state of development of the metastoma in eurypterids, and the higher specialization of the last pair of legs, it is inferred, that "Limulus represents a more primitive type in almost every respect except the fusion of the abdominal segments, and is to be related to the eurypterids 
Figure 27 Protolimulus eriensis Williams. Chemung group, Erie county, Pennsylvania. (From Hall and Clarke) not by direct descent, but through a comparatively unspecialized ancestor."
Our evidence is fully accordant with Laurie's view of the early separation of the eurypterids and Limulus. Of especial importance in this regard is the occurrence of a typical eurypterid in the Cambric era (Strabops). The ventral side of this form is still unknown and in view of its important bearing on the phylogenetic relations of the eurypterids with the limulids and scorpions, the elucidation of the character of its genital operculum is especially desirable. At any rate, none of the eurypterids now known from the Cambric to the Permic even suggest any fusion of the abdominal segments such as distinguishes Limulus, while in the Devonic Protolumulus eriensis Williams, the abdominal segments are apparently already fused and the characteristic Limulus structure developed.
With the conclusion that the limulids and eurypterids were probably separate in Precambric time, the question arises as to the date of separation of the Scorpionidae from the eurypterids. The relationship of Scorpio to Limulus has been fully discussed, especially by Lankester, and is no longer doubted. That of Scorpio to the eurypterids has been more fully
