336 SIDNEY F. HAKMER. 



wliat was once a completely calcified wall. The epithecal 

 investment is particularly obvious in Euthyris. It is 

 possible that the basal epitheca is a reminiscence of a time 

 when the frond was composed of two layers of zooecia, placed 

 back to back. I have previously (1901, p. 17) pointed out 

 that there is evidence that many Cretaceous Cheilostomes 

 were provided with a similar basal epitheca. 



(7) In the Microporelloid series the compensation-sac opens 

 by a '^ median pore," which has probably resulted from the 

 closure of a Schizoporelliform sinus. Tlus view, which has 

 been adopted by Gregory ^ as the basis of his division 

 Schizothyriata, receives support from the occasional occur- 

 rence in the group of an ancestrula with a Schizoporella- 

 like operculum (cf. p. 324). Gregory's classification places 

 Schizoporella with Microporella, and separates both 

 from Lepralia. It appears to me, on the contrary, that 

 Lepralia and Schizoporella cannot be separated from 

 one another, whereas Microporella is a distinct step in 

 advance of the other two. 



This section includes Microporella,, and perhaps "Cri- 

 brilina" radiata among encrusting forms; while Calwellia 

 (in which I place Urceolipora den tata), Ichthyaria, and 

 Onchoporella also belong to it. I think Miss Jelly " goes 

 too far in placing Siphonicy tara in Calwellia. Oncho- 

 porella may include O. borabycina, auctt., 0. ligulata, 

 Bask,'^ and O. selenoides, Ortmann,'^ but there seems to be 

 little except its habit to distinguish it from Calwellia. The 

 suture which in the latter genus connects the middle line of 

 the operculum with the median ]wrv probably indicates tlie 



' 1893, p. 224. 



2 1889, p. 33. 



^ ' Quart. Journ. Micr. Sri.,' viii, 1800, p. 282. Tins species has a close 

 resemblance to O. boiiibyci iia, auctt. It was clearly by an ovcrsii,'lit lliat 

 Busk (1881', p. 104, n.) referred Scrnparia d lap liana instead of Car base a 

 ligulata to his genus Onciioporelia ; tlie original figures of the two species 

 liaving appeared on the same plate of this Journal. 



* 'Arch. f. Naturg.,' ivi (i), 1890, p. 28. 



