COELENTERA — GRAPTOLITES. 



47 



reduced, e.g. Loganogra])t'iLS, Tetragraptus, until there arose a Gallery X. 

 form with only two saw-blades to a single handle, e.g. Didy- 

 mograptus. If these hung from sea- weed the polyps would be 

 mouth downwards, a position obviously ill adapted for 

 securing food-particles showered from the floating weed. 

 This was remedied in two different ways. In one way, the 

 two blades, instead of hanging down like a pair of tongs, 

 gradually opened until at last they were directed upwards. 

 In the other way, the polyps of the first two thecae seem to 

 have stretched upwards and so to have bent their thecae in 

 that direction ; the rest followed suit, and grew upwards 

 along the thread from which the sicula hung, e.g. Diplo- 

 graptus. Thus was formed quite a different type of graptolite. 

 Further developments took place in these latter : the colony 

 acquired a median supporting rod or virgula; this ended 

 often in a disc, which, it is supposed, was hollow, and served 

 as a float. The colonies were often compound, and many vir- 

 gulae with their thecae were attached to a single disc. If the 

 supposition that such forms took to a free-floating existence 

 be correct, we shall understand why succeeding forms, pre- 

 sumably their descendants, should have had their periderm 

 formed of a meshwork of fine strands, e.g. Betiolites : this is a 

 well-known way of obtaining lightness without loss of strength. 



Such are the main lines of graptolite evolution up to 

 their abrupt end at the close of the Silurian Epoch. But 

 there were many subsidiary lines ; and all these, combined 

 with the wide distribution of each successive form owing to 

 its floating life, have rendered the graptolites of enormous 

 value to the geologist in determining the succession of layers 

 in great thicknesses of rock, and in tracing those layers over 

 a large extent of country, even when much disturbed by 

 later earth-movements. A monograph of the British species 

 is in course of publication by the Palaeontographical Society. 



Fossils that can with certainty be referred to the 

 Hydrozoa are very few, and are not older than Cretaceous. 

 They are confined to forms in which the ectoderm secretes 

 outside the polyp or zooid a number of small calcareous 

 rods ; these grow together into a solid mass, leaving tubes in 

 which are the polyps. Most of these forms are generally 

 called Hydrocorallines, lately divided into Stylasterina and 

 Milleporina, of which only the latter are found fossil ; and, 

 indeed, Millepora itself is not known in rocks older than 

 some Pleistocene raised beaches. The Eocene fossil Axopora Table-ease 

 is but a doubtful ally. ^- 



