Development and Mode of Growth of Diplograptus. 237 



importance in the existence of the virgula in the sicula of Diplo- 

 graptus. But even with this difference the homology of the 

 sicula and the primary polypite of the calyptoblastic Hydrozoa 

 is such as to justify its being regarded as an indication of 

 relationship. 



Development of Diplogra^ptus pkistis, Hall. 

 Plate III. 



When the shale which contains the compound colonies of 2>. 

 pristis was exposed last year, I failed to pay sufficient attention 

 to collecting slabs with siculae. As I found later on, however, 

 some slabs which preserved sicula and young colonies with 

 attached base, I took advantage of a renewed but unfortunately 

 very brief exposure of the same layer, by the construction of a 

 road, to collect young colonies. The study of these tiny fossils 

 furnished the material for Plate III, in which I have arranged 

 the numerous various appearances of the young colonies of D, 

 pristis, Ilall, according to supposed successive stages of growth. 



Figs. 1-3 are representations of sicul^e with their basal ap- 

 pendages. The next stage are siculse with the first theca (figs. 

 4-7). The theca buds at the distal wide end of the sicula from a 

 round hole which is sometimes perceptible. The "connecting 

 canal " of Tornquist and the growth lines, described by Wiman, 

 could also be observed on some specimens. 



Figure 4 represents a specimen, 'the basal plate of which has 

 concave outlines, similar to fig. 2. The sicula is distinctly 

 attached to the central round node, as it is also in the following 

 specimens. 



Figure 5 belongs to a specimen in an excellent state of preser- 

 vation. The sicula is preserved as a tube and the basal append- 

 age as a segmental projection with a filiform chitinous border, 

 which is a section of the test. The central node lies on the top 

 of the projection. In fig. 6 the base is broken out, leaving a 

 deep square impression The central node is preserved as an 

 oval chitinous body with a central pit ; the latter probably repre- 

 sents a connection between the node and the basal cyst (cf. fig. 6a). 



Figure 7 is interesting in several regards. It not only shows 

 the "connecting canal" of the first theca and the hole from 

 which the former sprung, but also the central node on top 



