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ON DIPLOGRAPTIDÆ, LAPWORTH.
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opposite side of the hydrosoma. The alternating of the thecæ, therefore, is not only governed by the greater space attainable, but by the age and origin of the thecæ as well. In certain respects this law is also true for the scicula which may be regarded, if desirable, as the primary theca.

The growth lines meet on the outer edge of the hydrosoma in such a way as to produce a zig-zag line (Pl. II., fig. 7). This is probably produced by the same cause as the marking of the lobes of the scicula. Analogous to these, also, are the well-known paired spines of the thecal apertures of certain diplograptids, which are likewise an expression of the bilateral symmetry of the thecæ.

The partition wall between two adjoining or opposite thecæ is naturally double, and exhibits a slight thickening on the proximal inner edge.

The angle between the median line of the hydrosoma and the double partitions of the thecæ is greater in the distal portion, (25°-30°) than in the proximal, where it is occasionally zero.

An examination of figures 8 and 9 shows that the scicula orginally lies outside of the hydrosoma, except for the loose adherence of the thecæ, being united to it only at the termination of the first theca. Nevertheless the earliest thecæ are indented on the dorsal side and partly enclosing the scicula, so that it appears to lie in a depression of the outside of the hydrosoma. The thecæ extend more and more over the scicula until the central space is nearly transformed into a tube encircling the scicula, and when the fifth theca, i. e., the third on the same side as the first, finally opens, the scicula then disappears into the hydrosoma (Pl. II., fig. 9). The place where the scicula comes in contact with the perforated wall of the hydrosoma lies beneath the boundary between the two parts of the scicula (Pl. II., fig. 9).

The Virgula.As the virgula has been observed to occur within, and protrude from, both ends of the hydrosoma, it has been naturally concluded that the virgula passed without interruption through the entire hydrosoma. This, however, is not the case (Pl. II., figs. 1-3). The origin of that portion of the virgula which lies in the left wall of the proximal part of the

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