270 THE JOURNAL OF GEOLOGY. 



gination of the wall, and appear to develop like the spines of 

 moUusks. In one example (PI. 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 Diplographis. 



A second large hole opens on the first theca towards the front 

 side (PI. II., fig. 4). On the back side (PI. II., fig. 5), the lines 

 are disposed parallel with its margin ; on the front, however, 

 they converge. From this opening comes the second theca 

 (PI. 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 thecae 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 ori- 

 gin 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 (PI. II., fig. 8). 



In my material occur many specimens having only the scic- 

 ula, the first two thecae, 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 dis- 

 tal end. From it, the fourth theca takes its origin, and is situ- 

 ated in front of the scicula on the right side of the hydrosoma 

 (PI. II., fig. 8). 



Even if the openings between the first and second and the 

 second and third thecae 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 thecae 

 could be deduced : Each theca has its origin in the next on the 



