GRAPTOLITOIDEA 281 



this Medusa, like many others of the order, undergoes considerahle 

 changes in form before it reaches the sexually mature stage. 



Phialidium temporarium is one of the commonest Medusae of 

 our coast, and sometimes occurs in shoals. It seems probable that 

 it is the Medusa of Clytia johnstoni} By some authors the jelly- 

 fish known as Upenthesis is also believed to be the Medusa of a 

 Clytia. 



Fam. Dendrograptidae. — This family includes a number of 

 fossils which have certain distinct affinities with the Calypto- 

 blastea. In Dictyonema, common in the Ordovician rocks of 

 Norway, but also found in the Palaeozoic rocks of North America 

 and elsewhere, the fossil forms fan -shaped colonies of delicate 

 filaments, united by many transverse commissures, and in well- 

 preserved specimens the terminal branches bear well-marked 

 uniserial hydrothecae. In some species thecae of a different 

 character, which have been interpreted to be gonothecae and 

 nematophores respectively, are found. 



Other genera are Dendrograptus, Thamnograptus, and several 

 others from Silurian strata. 



Order V. Graptolitoidea. 



A large number of fossils, usually called Graptolites, occurring 

 in Palaeozoic strata, are generally regarded as the skeletal remains 

 of an ancient group of Hydrozoa. 



In the simpler forms the fossil consists of a delicate straight 

 rod bearing on one side a series of small cups. ' It is suggested 

 that the cups contained hydroid zooids, and should therefore be 

 regarded as the equivalent of the hydrothecae, and that the axis 

 represents the axis of the colony or of a branch of the Calypto- 

 blastea. In some of the forms with two rows of cups on the 

 axis {Diplograptus), however, it has been shown that the cups are 

 absent from a considerable portion of one end of the axis, and 

 that the axes of several radially arranged individuals are fused 

 together and united to a central circular plate. Moreover, there 

 is found in many specimens a series of vesicles, a little larger in 

 size than the cups, attached to the plate and arranged in a circle 

 at the base of the axes. These vesicles are called the gonothecae. 



The discovery of the central plate and of the so-called gono- 

 ' E. T. Browne, Bergens Museums Aarlog, 1903, iy. p. 18. 



