Page:A Revision of the Families and Genera of the Stylonuracea (Eurypterida).djvu/7

Fig. 95. Schematic diagrams of the carapaces of the genera of the families Stylonuridae, Pageidae, Drepanopteridae, Kokomopteridae, Rhenopteridae and Laurieipteridae. 1, Stylonurus; 2, Parastylonurus; 3, Stylonuroides; 4, Stylonurella; 5, Brachyopterus; 6, Brachyopterella; 7, Clarkeipterus; 8, Melbournopterus; 9, Kokomopterus; 10, Pagea; 11, Drepanopterus; 12, Rhenopterus; 13, Laurieipterus; 14, Ctenopterus; 15, Hallipterus; 16, Mazonipterus.

Rhenopteridae, it is obvious that many of the species presently placed in the family Stylonuridae differ widely and show so little affinity that continued grouping under a single family (or mainly, a single genus) is not consistent, judicious or desirable. In the Scorpionida, the sternum, the structure homologous with the eurypterid metastoma, is of great phylogenetic and taxonomic importance for the separation of families (see Petrunkevitch, 1916, pp. 600–608). We have in the past used the ventral shield and metastoma as essen-