Q2 NEW YORK STATE MUSEUM 



bodies to broad ovate or subcircular ones in mature rhabdosomes, independ- 

 ently from the growth-stages of the latter. From these facts we infer that 

 the expansions were a secondary development, and not homologous to the 

 "primary disk" from which the siculae and colonies of most graptolites are 

 suspended; but perhaps in some measure corresponding to the "central 

 disk" of certain Dichograptidae. 



2 Certain observations suggest that the appendages were indeed bags 

 or vesicles and not flat "disks" or "wings." These are the distinct over- 

 lapping of some parts over others in the compressed state [text fig. 25], the 

 presence of longitudinal folds and the collapsed appearance of some of the 

 appendages and the composition of the test of two superposed layers which 

 is indicated by a partial separation of the layers, apparently from a bursting 

 of the body [see text fig. 26] and the breaking up of the vesicle into three 

 or four broad widening longitudinal bands [see Barrande's figure of D . 

 pal me us] or into two equal overlapping halves as in Nicholson's specimen 

 of D i p 1 o g r a p t u s folium [see text fig. 1 9]. 



3 Two possible functions of these vesicles have been tentatively sug- 

 gested, viz, their connection with the former presence of older thecae by 

 Barrande [see above | and their possible service as pneumatophores in cases 

 of accidental separation of the rhabdosomes from the colony [see Mem. 7, 

 p. 519]. Barrande's view is based on a wrong conception of the rhabdo- 

 some, the latter being now known to have grown in the direction of the 

 vesicle. The writer had stated in the afore mentioned place that " the 

 vesiclelike expansions of the nemacaulus and of the distal appendages of 

 certain species of Diplograptus aie plausibly explained as organs which 

 assisted in floating the synrhabdosomes and eventually protected the broken- 

 off rhabdosomes from sinking." In the same volume [p.652] a like origin 

 is assigned to the broad nema of Tetragraptus fruticosus. 



As observations in favor of the latter view could be cited the apparent 

 vesicular shape of the appendage, its development in mature rhabdosomes 

 only, where the weight is greatest. Perhaps in such species as Diplo- 

 graptus vesiculosus, where they are said to be always present, they 



