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THE JOURNAL OF GEOLOGY.

gination of the wall, and appear to develop like the spines of mollusks. In one example (Pl. II., fig. 6), two approximate spines are joined by a thin skin. Therefore there are from four to six spines on the proximal end of this Diplograptus.

A second large hole opens on the first theca towards the front side (Pl. II., fig. 4). On the back side (Pl. II., fig. 5), the lines are disposed parallel with its margin; on the front, however, they converge. From this opening comes the second theca (Pl. II., fig. 7), on the right side in front of the scicula. Shortly after it has left the first theca, the turning of the hydrosoma must have taken place; i. e., both thecæ now begin to grow in an opposite direction and thereby change the direction of the aperture towards the distal end. Further scicula growth probably ceases at this stage of development. The newly-formed theca fastens itself to the forward spine on the right side of the scicula.

The second theca hardly has left the first when it gives origin to the third, which also lies on the first, and is therefore situated on the left side. The pore uniting the third theca with the second is situated a little more proximally than that which joins the second with the first. The earliest budding of the third theca, therefore, occurs between the origin of the second and the turning of the hydrosoma. Its lower part fills the space in the bend of the first theca (Pl. II., fig. 8).

In my material occur many specimens having only the scicula, the first two thecæ, and the proximal portion of the third. If such an example were pressed flat without relief, and changed into pyrite, it would be recognized as a scicula having two buds with a common canal. The third theca increases only at the distal end. From it, the fourth theca takes its origin, and is situated in front of the scicula on the right side of the hydrosoma (Pl. II., fig. 8).

Even if the openings between the first and second and the second and third thecæ were not apparent, but only the origin of the first theca from the scicula and the fourth from the third were observed, the following law for the formation of the thecæ could be deduced: Each theca has its origin in the next on the

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