West & West. — Observations on the Conjugatae. 55 
were probably developed, and at a short period before the 
Zygnemeae became differentiated into the two distinct groups 
represented by Spirogyra and Zygnema , the Desmidieae were 
probably evolved by retrogression. The view that the latter 
may have been evolved in this way is confirmed by the 
occasional reversion of Hyalotheca to its ancestral mode of 
conjugation, the remains of which are still found in Desmidium 
Table of Phylogeny. 
cylindricum . Pleurodiscus , which conjugates like a Zygnema , 
was probably evolved from the Zygnema group just after 
its differentiation from the Spirogyra group, Pyxispora 
being evolved from Zygnema by the assumption of a rudi- 
mentary sporophyte-generation, thus placing it on an equal 
level of specialization with the Mesocarpeae. Zygnema can 
be connected with Debarya by the subgenus Zygogonium , and 
