Development and Mode of Growth of Diplograptus. 223 



along the axes, but suggests, at the same time, that it may have 

 served still other purposes of the animal economy. In concord- 

 ance with the first supposition is his observation that the 

 central disc is found where the divisions at the base become more 

 numerous, while it is absent in some of the four-stiped forms ; 

 but on the other hand, its " greater or less development is not 

 always corresponding to the size and extent of the stipes." 

 Huxley has compared this organ with the basal plate of Defran- 

 cia, a Bryozoan, while Nicholson thinks it to be homologous to 

 the " float or pneumatocyst " of the Physophoridce, an order' of 

 the Sijphonophora. 



The central disc of the two species of Diplograptus which I 

 have before me appears as a nearly square chitiaous plate, some- 

 times drawn out a little at the corners. It is relatively small in 

 regard to the diameter of the whole colony, but must have been 

 a ^ery strong and solid organ as it is mostly distinct, even in 

 poor specimens, where other organs can not be distinguished. 

 That this plate is formed by two laminaa is demonstrated by 

 such specimens as reproduced in PL I, figs. 4, 6, 8; PL II, 

 fig. 4, where the raised edges and the depressed middle part of 

 the organ prove that it has been burst open. Especially in the 

 specimen represented in fig. 8, the central disc is preserved so 

 distinctly in relief that it can easil}/ be studied with the naked 

 eye. It is here a deep concave chitinous trough, inside of which 

 lie the funicle and the bases of the branches. In PL I, fig. 4, 

 we are able to see that the axes, which here give unmistakable 

 evidence of having been canals, pierce the vesicle which incloses 

 the funicle. Where the central disc is not broken, as in PL I, 

 fig. 7, its upper side is convex. Funicle and central disc have 

 in our species similar forms, parallel margins and their diagonals 

 coincide ; the funicle is always distinctly inclosed in the central 

 disc. 



The form, solidity and connection of the central disc with the 

 stipes agree with Hall's suggestion that this organ served to 

 support the bases of the* stipes. It was also certainly a protec- 

 tion to the funicle, but probably these were not its only func- 

 tions. Nicholson's supposition that it was a " float" seems very 

 acceptable indeed, if we regard the large central discs of some 



