3. The type of Thelyphonus, Hypoctonus, Phrynus, and other members of the Pedipalpi, in which the operculum forms a part of the ventral surface of the body wall, but no longer covers only the termination of the genital apparatus. It really consists of two parts, a median anterior, which covers the terminal genital apparatus, {190}and a lateral posterior, which covers the first pair of gills, or lung-books, as they are called. In this type (Fig. 77) the genital ducts terminate in a common chamber or uterus, the nature of which will be further considered. As has been pointed out by Blanchard, the terminal genital organs of the scorpions and the Pedipalpi vary considerably in the different genera, especially the male genital organs. The general type of structure is the same, and consists in both male and female of vasa deferentia, which come together to form a common chamber before the actual opening to the exterior. This common chamber has been called in the female scorpion the vagina, or in Thelyphonus the uterus. I shall use the latter term, in accordance with Tarnani's work, and the corresponding chamber in the male will be the _uterus masculinus_. A considerable discussion has taken place about the method of action of the external genital organs in the members of the scorpion tribe, into which it is hardly necessary to enter here. The evidence points to the conclusion that in all these forms the operculum covers a median single chamber or uterus, into which the genital ducts open on each side, the main channels of emission being provided with a massive chitinous internal framework. We may feel certain that in the old extinct sea-scorpions, Eurypterus, etc., a similar arrangement existed, and that therefore in them also the median portion of the operculum covered a median chamber or uterus composed of the amalgamation of the terminations of the two genital ducts, which were originally separate, as in Limulus. [Illustration: FIG. 77.--OPERCULUM AND FOLLOWING SEGMENTS OF MALE THELYPHONUS. Opercular segment is marked out by thick black line. _Ut. Masc._, uterus masculinus; _Int. Op._, internal opening of uterus into genital chamber; _Ext. Op._, common external opening to genital chamber (_Gen. Ch._) and pulmonary chamber.]