The observations of Schmidt, Zittel, and others show that the {191}operculum in the old extinct sea-scorpions, Eurypterus, Pterygotus, etc., belonged to the type of Thelyphonus, rather than to that of Limulus or Scorpio. In Fig. 78 I give a picture from Schmidt of the ventral aspect of Eurypterus, and by the side of it a picture of the isolated operculum. Schmidt considers that there were five branchiæ-bearing segments constituting the mesosoma, the foremost of which formed the operculum. Such operculum is often found isolated, and is clearly composed of two lateral appendages fused together in the middle line, of such a nature as to form a median elongated tongue, which lies between and separates the first three pairs of branchial segments. This median tongue, together with the anterior and median portion of the operculum, concealed, in all probability, according to Schmidt, the terminal parts of the genital organs, just as the median part of the operculum in Phrynus and Thelyphonus conceals the complicated terminal portions of the genital organs. The posterior part of the operculum, like that of Phrynus and Thelyphonus, carried the first pair of branchiæ, so Schmidt thinks from the evidence of markings on some specimens. [Illustration: FIG. 78.--_Eurypterus._ The segments and appendages on the right are numbered in correspondence with the cranial system of lateral nerve-roots as found in vertebrates. _M._, metastoma. The surface ornamentation is represented on the first segment posterior to the branchial segments. The opercular appendage is marked out by dots.] Apparently an opercular appendage of this kind is in reality the result of a fusion of the genital operculum with the first branchial appendage in forms such as the scorpion; for, in order that the tergal plates may correspond in number with the sternal in Eurypterus, etc., it is necessary to consider that the operculum is composed of two sternites joined together. Similarly in Thelyphonus, Phrynus, etc., this numerical correspondence is only observed if the operculum is looked upon as double. A restoration of the mesosomatic region of Eurypterus, viewed {192}from the internal surface, might be represented by Fig. 79, in which the thick line represents the outline of the opercular segment, and the fainter lines the succeeding branchial segments. The middle and anterior part of the opercular segment carried the terminations of the genital organs; these I have represented, in accordance with our knowledge of the nature of these organs in the present-day scorpions, as a median elongated uterus, bilaterally formed, from which the genital ducts passed, probably as in Limulus, towards a mass of generative gland in the cephalic region, and not as in Scorpio or Thelyphonus, tailwards to the abdominal region. [Illustration: FIG. 79.--DIAGRAM TO INDICATE THE PROBABLE NATURE OF THE MESOSOMATIC SEGMENTS OF EURYPTERUS.