Development and Mode of Growth of Diplograptus. 235 



primary thecae and found that those of Dijplograjptus throw off 

 buds from approximately opposite sides near the broad end, and 

 form thus the two series of theca3 characteristic of the Dip- 

 lograptidae. He stated that the sicula remains embedded in 

 the so-called proximal end of the rhabdosome, projecting with 

 its broad end, and that the virgula grows out to the central axis. 



An interesting contribution to our knowledge of the sicula of 

 Di])lograptus^ has been lately furnished by Carl Wiman."^ The 

 author found in a piece of limestone, from Bornholm, half car- 

 bonized chitinous siculae, which he was able to make transparent 

 by means of Schulze's maceration medium. With the aid of 

 his excellent material he was enabled to state that the sicula has 

 the form of a conical tube and consists of two parts, a very thin 

 and transparent "distal," and a thicker and less transparent 

 "proximal" one. 



My material has not given me any opportunity to study the 

 details of this cone, but has furnished valuable information 

 regarding the relation of the sicula to the complete colony. 



While the profuse covering of certain slabs with siculse and 

 the crowding of siculae around some colonies (PI. I, fig. 9) give 

 conclusive evidence that numerous siculas left the gonangia, it is 

 suggested by siculae and by very young stipes attached to adoles- 

 cent colonies that some siculaB did not sever their connection with 

 the parent colony but grew out into new rhabdosomes. The 

 original of PI. I, fig. 5, shows one sicula (si) with two thecas and 

 a young rhabdosome (sg) with a still distinctly visible sicula. 

 Young colonies bear whole bundles of siculae (PL III, figs. 15, 16). 



The free sicula generally bears only a very delicate, filiform 

 process, like that by which it is attached to the blastostyle. 

 After I had found that this process on young free rhabdosomes 

 connects with a square chitinous plate, I also examined the 

 free siculae for these appendages and found them in fair number 

 (PI. Ill, figs. 1-3) even on some of the siculae which apparently 

 had only shortly before been liberated (PL I, fig. 9). On the last- 

 mentioned specimen they are visible only outside of the dense 

 crowd of siculae, the latter obscuring the details about the center. 

 The study of this specimen leads to the conclusion that the 



* Ueber Diplograptidee Lapw. Bull, of the Geol. Inst, of Upsala, vol. I, No. 2, 1893, Translated by 

 Ch. Schuchert In the Journal of Geology, 1894, p. 267. 



