Page:Pennsylvanian invertebrates of the Mazon Creek Area, Illinois Eurypterida.djvu/20

 . 47. Well-preserved specimen (male; Type A) of Adelophthalmus mazonensis (Meek and Worthen), PE 5094; × 1.5. The underlying mesial appendage was developed to reveal the lateral lobes of the operculum shown on figure 48.

lobes are large, ear-shaped, and very similar to those of Type B (see fig. 48). This is an important morphological structure, as the operculum of Adelophthalmus mansfieldi (C. E. Hall) is quite different with regard to the spatulate lobes. The Type A mesial appendage of A. mazonensis, although not clearly preserved, is considerably larger than in A. mansfieldi. The operculum of A. mansfieldi is given here for comparison (see fig. 51). This is James Hall's hypotype (1884, fig. 4), which remains the only known well-preserved Type A operculum of the genus Adelophthalmus.

A very poorly preserved specimen, PE 6174, paradoxically represents the only specimen in which the alimentary canal is preserved. This is preserved from the second tergite to about the ninth, and appears to be thickest in the area occupied by the fourth, fifth, and