202 COELENTERATA HYDROZOA chap. 



thecae suggests that the usual comparison of a Graptolite with a 

 Sertularian Hydroid is erroneous, and that the colony or indi- 

 vidual, when alive, was a more or less radially symmetrical 

 floating form, like a Medusa, of which only the distal appendages 

 (possibly tentacles) are commonly preserved as fossils. 



The evidence that the Graptolites were Hydrozoa is in reality 

 very slight, but the proof of their relationship to any other 

 phylum of the animal kingdom does not exist.'- It is therefore 

 convenient to consider them in this place, and to regard them, 

 provisionally, as related to the Calyptoblastea. 



The order is divided into three families. 



Fam. 1. Monoprionidae. — Cups arranged uniserially on one 

 side of the axis. 



The principal genera are Monograptus, with the axis straight, 

 curved, or helicoid, from many horizons in the Silurian strata ; 

 Rastrites, with a spirally coiled axis, Silurian ; Bidymograptits, 

 Ordovician ; and Coenograptus, Ordovician. 



Fam. 2. Diprionidae. — -Cups arranged in two or four vertical 

 rows on the axis. 



Diplograptus, Ordovician and Silurian ; Cliviacograptus, Ordo- 

 vician and Silurian ; and Phyllograptus, in which the axis and 

 cups are arranged in such a manner that they resemble an ovate 

 leaf 



Fam. 3. Retiolitidae. — Cups arranged biserially on a reticu- 

 late axis. 



Betiolites, Ordovician and Silurian; Stomatograptus, Retio- 

 graptus, and Glossograptus, Ordovician. 



Fossil Corals possibly allied to Hydrozoa. 



Among the many fossil corals that are usually classified with 

 the Hydrozoa the genus Porosphaera is of interest as it is often 

 supposed to be related to Millepora. It consists of globular 

 masses about 10-20 mm. in diameter occurring in the Upper 

 Cretaceous strata. In the centre there is usually a foreign body 

 around which the coral was formed by concentric encrusting 

 growth. Eunning radially from pores on the surface to the 

 centre, there are numerous tubules which have a certain general 

 resemblance to the pore-tubes of Millepora. The monomorphic 



^ Cf. Sohepotieff, Neues Jahrh. f. Mineralogie, 1905, ii. pp. 79-98. 



