160 F Cavers. 
and when the stalk lengthens these scales are carried up with the 
receptacle. The whole structure is apparently a mere dorsal out¬ 
growth of the thallus tissue, the apical growth of the thallus not 
being arrested by the development of a receptacle. The receptacle 
of Clevea may therefore be compared with the outgrowth which 
Fig. 18. Astropora. —I, Clevea hyalina ; II, Sauteria alpina ; III , Peltolepis 
grandis. 
arises among the archegonia in Corsinia, but in Clevea the archegonia 
do not appear until the outgrowth has developed into a nearly 
hemispherical structure, about 0-1 mm. in diameter, and when they 
do appear they arise from the tissue of the outgrowth itself. 
In Sauteria the receptacle begins in the same way as in Clevea , 
but the apical growth of the thallus is arrested when a receptacle 
is formed, hence a branch of the thallus bears a single receptacle 
only. According to Leitgeb, a single archegonium is present in 
each involucre and a single groove in the stalk. The writer found, 
