288 Graptolites. 



describes and figures the axis in D. folium, His., as lateral, or 

 attached to one of the sides, while the coenosarc was con- 

 tinuous between it and the other side. A different structure 

 has been observed by Hall, who has proposed to separate the 

 species of which it is a characteristic into a distinct genus 

 Climacograptus. In this group the polypites were not lodged 

 in distinct hydrotheca3, but the cells containing them were^ 

 enclosed in the continuous chitinous polypary, which was 

 pierced at regular intervals by openings for the egress of the 

 animals (Figs. 4 and 13). The axis is filiform and central, and 

 the triangular plate, which partially separated the adjoining 

 polypites, rises from a point in the axis, and widens as it 

 passes upwards and outwards, until it reaches the outer surface 

 of the polypary. A free space would thus be left on either 

 side of the axis for the common coenosarc. This axis, in the 

 double graptolites, appears to have been double — one half 

 belonging to each of the two series of polypites — for it is 

 sometimes seen to separate into two divisions after it passes 

 beyond the polypiferous portion of the organism, and this is 

 further confirmed by the structure of a sub-genus (Dicrano- 

 gvaptus) of Climacograptus. In this the older portion of the 

 polypary has a double series of cells, but it speedily divides 

 into two branches, each of which agrees in structure with the 

 genus Graptolithus . The internal axis of the older portion 

 breaks up into the external axes of the two branches. 



Two remarkable genera of double graptolites, belonging 

 perhaps to a different family, must be noticed here. The one, 

 Retiolites, Barr. (Fig. 12), is without a central axis, and the two 

 series of cells rise on either side of a single internal canal, 

 which occupies the central portion of the polypary. The other, 

 Pltyllograptus, Hall (Figs. 5 a and b) has a solid central axis, 

 but is destitute of a common canal, the plates of the different 

 cells being continued to the solid axis. 



The presence of a solid axis in all true graptolites deserves 

 special notice. In several, if not in all the species of the 

 family, it is continued naked beyond the growing portion of 

 the polypary. How far it extended, or what was the nature of 

 its termination, has not yet been ascertained. In D. pristis, I 

 have noticed it more than three inches long. In a mass of 

 specimens of this species, which I found on one slab, the naked 

 axes were beautifully preserved, and the individual specimens 

 were arranged so that they all radiated from a small space in 

 which the axes met. I could not, however, trace any connec- 

 tion between the different axes, but it was evident that they 

 had become entangled while they were yet fresh — if not 

 actually living. 



The appearance on this slab did not in the least correspond 



