Development and Mode of Growth of Diplograptus. 241 



colonies are most probably the relics of already farther developed 

 colonies. There are a few other relics of colonies with a strange 

 appearance, as for instance a central disc bearing one large 

 rhabdosome and one sicula. 



The following is the development of Diplograptuspristis^ Hall, 

 given in a more comprehensive form : 



1. The detached sicula is provided with a basal appendage, 

 to which it is attached by means of a little round node. 



2. The node becomes the central disc and funicle. The sicula 

 produces at first one theca, then a second, a third, etc. 



3. With the budding of the first thecae, the growth of the 

 gonangia already begins, with usually four small oval capsules. 



4. The further growth is marked by a remarkable lengthening 

 of the hydrocaulus and a continued budding of thecae at the 

 proximal end of the primary rhabdosome, along the hydrocaulus 

 and toward the center. 



5. At last the gonangia mature and open. Many, or perhaps 

 all of the siculae, remain connected to the parent colony. Tlje 

 colony consists now of a rhabdosome, about half developed, which 

 is the primary one, bearing a basal cyst, central disc and funicle, 

 and on these, bundles of siculae. 



6. These siculse grow out to rhabdosomes, the process begin- 

 ning again with a lengthening of the hydrocauli. 



7. After this first generation of rhabdosomes has reached a 

 certain age, a second generation of gonangia begins to grow. 



8. These latter open again and produce a new set of siculae 

 around the center. The colony consists now of the primary 

 rhabdosome, a verticil of young rhabdosomes, and another of 

 siculae. This process is continued, the successive generations of 

 gonangia producing siculae, which, in turn, develop into verticils 

 of rhabdosomes. 



The different size of the rhabdosomes, in all hydrosomes appa- 

 rently still growing (PL I, fig. 1) ; their equal length in the hydro- 

 somes with the longest and most numerous rhabdosomes; the 

 common occurrence of one mature rhabdosome and numerous 

 young ones in the same colony, are all facts giving evidence that 

 the hydrosome began to grow with one rhabdosome, and that the 

 number and length of the rhabdosomes increased with the age of 

 the whole colony. 



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