154 J. HOPKINSON ON BRITISH GRAPTOLITES. 



From the radicle proceeds the solid axis. With reference to the 

 CEenosarc and pol}^3ites it is dorsal in position,* being placed in a 

 hollow at the back of the ca^nosarc, and outside the protecting 

 periderm, from which it is easily separable. In form it is usually 

 solid and cylindrical ; sometimes it is composed of two thin lamina ; 

 and in one genus it is cruciform. It gave strength and support to 

 the whole polypary. It is frequently prolonged beyond it, and has 

 a similar horny structure. 



Attached to the solid axis is the periderm, containing the cylindri- 

 cal common canal. The cfenosarc which it enclosed has perished. 



On the margin of the periderm the hydrothecae are developed. Be- 

 tween them, and the common canal, there is no constriction or septum, 

 showing that the individual polypites were immediately developed 

 from the csenosarc, the whole being organically united ; each in- 

 dividual polypite contributing, through the csenosarc, to the nourish- 

 ment of the whole colony. 



The entire polypary consists of a flexible horny or chitinous 

 cuticle, usually, in the process of mineralisation, carbonized, or 

 metamorphosed into iron pyrites. 



The hydrothecfe are more or less in contact throughout their 

 length, or are entirely separated. When in contact, they usually 

 form an acute angle with the axis ; when separated, they are often 

 rectangular to it. In some peculiar forms there are no distinct 

 hydrothecfe, their orifices being excavated in the margin of the 

 polypary, which is continuous throughout. The hydrothecae, in 

 these forms, are in contact for their whole length, and have but a 

 single wall of division between them ; while, in the ordinary forms, 

 each is perfect in itself. 



The hydrothecEe may be developed in one or more series from the 

 axis. When in one series, the axis is round ; when in two, they 

 are disposed on each side of a double axis, which is sometimes flat- 

 tened ; when in four, they are arranged in a cruciform manner 

 round a cruciform axis. Their form and position are very varied ; 

 but we can, I think, collect the various forms into five types — linear, 

 curvilinear, tubular, rectangular, and incised. 



Under the term linear I include those long, narrow hydrothccse, 

 which are entirely separated fi'om each other (PI. viii., /. 1). The 

 term curvilinear is intended to take in those bounded by curved 



* In graptolites with a double series of hydrothecae it is dorsal to each series. 



