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ON DIPLOGRAPTIDÆ, LAPWORTH.
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line for line until the virgula begins to show, and practically absorbs the lines. The angle at which the lines and the virgula unite diminishes with age. The mature scicula is provided with three spines at its aperture, one of which is the cylindrical virgula. The other two are flat, and should probably be termed lobes. They have that appearance, as shown in figure 5, and are situated opposite the virgula on each side of a shallow emargination of the aperture, joined by a slight swelling of the border of the indentation.

The form and completion of the aperture gives the scicula a particularly conspicuous bilateral symmetry, on account of which the animal at this stage recalls a bryozoan rather than a modern hydroid polyp.

The Theca.Before the scicula has matured, there forms, at the point illustrated in Pl. II., fig. 4, the beginning of a second tube, which is also provided with growth lines. The circular perforation by which it communicates with the cavity of the scicula has been observed in forty-one examples. This opening is not produced by absorption of the wall as shown by the slight irregularity of the scicular growth lines at the origin of the second tube (Pl. II., fig. 4). This tube does not develop into a general canal or similar part, but forms the first theca. Then from this one chamber of habitation there simply develops a second.

The first theca (Pl. II., figs. 4, 5) at once leans closely on the scicula, widens very rapidly, approaches toward the virgula, and in bending around the scicula passes it, so that the theca comes to lie on the back (dorsal) side[1] of the scicula, and eventually both increase at an equal rate towards the proximal end. The growth lines of the first theca like those of the scicula, although in a less degree, are also drawn along, so to speak, by the virgula. As soon as the theca, which clings closely to the virgula, has grown a little further than the scicula towards the proximal end, it again changes its direction, and bends outward and eventually upwards. Where it begins to grow upwards, it gives off from one to three spines in succession, which start with a slight emar-

  1. ↑ I have named that side the front which in figures 4 and 7, faces the observer.
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