Fig. 115.—Dorsal half of Head-region of Ammocœtes. "Tr.", trabeculæ; "Pit.", pituitary space; "Inf.", infundibulum; "Ser.", median serrated flange of velar folds. (All references in this work to "Ammocœtes" or "Petromyzon" appear to refer to the species now called "Lampetra planeri".)