Development and Mode of Growth of Diplograptus; 229 



the fourth (right-hand side) is a corresponding impression. 

 They are surrounded by a deep furrow, from which, in one of the 

 gonangia (g), the surface rises in a projecting ring. The latter 

 is apparently the section of a central vesicle which did not con- 

 tain any solid substance at the time of the burial of the colony, 

 therefore has been filled with sediment and is now preserved as 

 the solid central node, whereas the test of the vesicle became 

 flattened and separated from the matrix. Outside the central 

 node we see radiating siculae. Near the left gonangium is a 

 group of impressions of siculae which apparently sprung from it. 

 The right gonangium shows radial and concentric wrinkles, the 

 former of which are probably impressions of sicul^, the latter 

 may have been wrinkles of the gonangial test. 



Another specimen which aids essentially in obtaining a com- 

 plete conception of these organs is reproduced in PI. I, fig. 9. It 

 shows seven, more or less oval, not very, distinct impressions of 

 gonangia, and is remarkable for the multitude of siculae. covering 

 the slab in the near neighborhood of the center. The position 

 of these siculae gives evidence that they came from the center of 

 the colony, and were apparently set free shortly before the bury- 

 ing of the colony by sediment. 



I have before me a great number of complete colonies with dis- 

 tinct gonangia. The number of the latter organs ranges from 

 four to eight, the majority of the hydrosomes, especially the 

 younger ones, bearing only four gonangia. 



In older colonies the chitinous test is rarely distinctly perceptible 

 because of the crowding of the hydrocauli and rhabdosomes 

 toward the center. Only in specimens like that represented 

 in PL II, fig. 4:, where the parts above the gonangia are 

 broken away, are the test and form of the gonangia clearly 

 visible. Young colonies, in which the center is less obscured by 

 overlying rhabdosomes, show the still closed gonangia as oval, 

 concentrically wrinkled, chitinous plates, (cf. PI. Ill, figs, log, 

 20g, 21g, and the chitinous rings in fig. 24, which probably are 

 remains of gonangia.) 



Some specimens of D. Euedemanni, (PI. II, ^g. 3g), show 

 a very nice preservation of the form of the gonangia. The 

 proximal parts of the rhabdosomes are covered by round plates 

 which have somewhat raised edges and a lighter color than the 



