FOSSIL CORALLINES. 43 



are all that is left of the living colony of which they 

 formed the more solid and endurable parts. The 

 Graptolites also were like the Sertularians in being 

 compound animals, or rather, a colony of simple, 

 hydra-like creatures ; whence the name of Hydrozoa. 

 In the Graptolites, however, the horny cups are 

 crowded closely together, so that they are all in con- 



Fig. 31. Twin Graptolite (Didymograptus Murchisoni). 



tact (Fig. 32), whereas in the modern Sertularians 

 they are distinct. In one genus of Graptolites, how- 

 ever, the cups are separate, and from the resemblance 

 they have to the teeth of a rake (Latin raster, a 

 rake), these forms go by the name of Rastrites. They 

 are usually coiled up like toothed watch-springs, 



