282 COELEXTERATA HYDROZOA chap. 



thecae suggests that tlie usual coniparison 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 sliglit, but the proof of their relationship to any other 

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

 convenient to consider tliem in tliis 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 Monograptvs, witli the axis straight, 

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

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

 Ordovician ; and Coenograpt^ts, Ordovician. 



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

 rows on the axis. 



Dijylogrcqjtus, Ordovician and Silurian ; Climacograptus, Ordo- 

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

 cups are n.rranged in such a manner that they resemble an ovate 

 leaf. 



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

 late axis. 



Retiolites, 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 Forosphaera is of interest as it is often 

 supposed to be related to MiUejyora. It consists of globular 

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

 Cretaceous strata. In tlie 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 wliich have a certain general 

 resemblance to the pore-tubes of Millcpora. The monomorphic 



^ Cf. Schepotieff, Xeurs Jahrh. f. Mineral oijU, 1905. ii. pp. 79-98. 



