ON DIPLOGRAPTID^, LAPWORTH. 269 



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 vir- 

 gula. 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 emar- 

 gination 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 PI. II., fig. 4, the beginning of a second tube, 

 which is also provided with growth lines. The circular perfora- 

 tion by which it communicates with the cavity of the scicula has 

 been observed in forty-one examples. This opening is not pro- 

 duced by absorption of the wall as shown by the slight irregu- 

 larity of the scicular growth lines at the origin of the second 

 tube (PI. 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 (PI. 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^ 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- 



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



