igo NATURAL SCIENCE. September, 



Holm, according to my view, and shall subsequently show how 

 Tornquist differs from me. 



Here, too, as appears to be the case with all Graptoloidea, the 

 sicula consists of the two essentially different parts, the initial part, 

 that is prolonged into a virgula, and the aperture-part, with its 

 mouth-spine. In Diplogvaptiis sp. (Figs. 3, 4, 5, 6), the sicula-mouth 

 is also provided with two paired lobes, whereas in Climacogmptus 

 hichersianus (Figs. 7 and 8) it is more like the sicula in Monograptus 

 dnhiiis. In both it is bilaterally symmetrical. From the anti -sicula 

 side of the sicula springs the first theca (Fig. 4), and immediately 

 bends sideways past the mouth-spine over to the sicula side. Here 

 it lies alongside the apertural end of the sicula, which, of course, had 

 the start of it, and the two grow in the same direction. This first 

 theca lies in the plane of symmetry of the sicula, on the same side as 

 the mouth-spine. Such is also the case in Monograptus dubius, 

 although the first theca there grows from the beginning in the opposite 

 direction to the apertural end of the sicula (Figs, i and 2). To return 

 to the Diplograptidae : the first theca, having reached as far as the 

 mouth of the sicula, begins to grow in another direction, bending out- 

 wards and upwards, towards the distal end of the rhabdome, either 

 much, as in Climacograptns, or little, as in DiplogvapUis (Figs. 5 and 6). 

 Before this change in the direction of growth has taken place, and 

 immediately after the first theca has left the sicula, it in its turn 

 gives origin to the second theca, which crosses over and places itself 

 on the opposite side of the sicula (Figs. 3, 5, 6, and 7). This second 

 theca again sends out the third theca, which places itself on the same 

 side as the first. 



It has long been known that certain Diplograptidae are divided 

 by a longitudinal septum. Figs. 7 and 8 show the origin of such a 

 septum. The third theca (i^^) sends out two thecae : first, one (^4) 

 towards the side of the second theca, and then one (4) towards 

 the side of the first theca. The septum seems to originate in the 

 same way, even when it begins further from the sicula end. When 

 there is a longitudinal septum, the thecae do not seem to be placed 

 alternately to the same degree as when it is wanting. This is easily . 

 understood ; for if there is no longitudinal septum, each theca springs 

 from the opposite side of the adjacent proximal theca ; where, on the 

 contrary, there is a longitudinal septum, then in those regions of the 

 rhabdome where the septum is, each theca arises from the adjacent 

 proximal theca, and on the same side as it (Fig. 7). 



In my opinion, the whole rhabdome thus consists of a colony of 

 animals, in which each individual has budded out from the next 

 eldest. Tornquist, however, contents himself with describing and 

 naming the cavities that are surrounded by the periderm, leaving 

 their nature undecided. The part of the first theca that lies between 

 the passage to the sicula and the passage to the second theca [a in 

 Fig. 3) is called by him the " connecting canal." That which was of 



