THE JIOHPHOLOGY OF THE CH15ILOST0MATA. 331 



tlie existence of a single comniunicatiou between the living 

 tissue of the frontal wall and that of the bocly-cavity, corre- 

 sponding with the base of the scutum. 



The group is usually characterised by the high develop- 

 ment of avicularia and vibracula. 



(3) The Microporoid series includes Micropora^ Tliala- 

 moporella, Diploporella^^ and Steganoporellaj all 

 characterised by the development of a cryptocyst" or calcareous 

 lamella which grows beneath the frontal membrane. In the 

 forms which have been examined, the parietal muscles are 

 i-educed to a single pair of highly developed depressor 

 muscles which commonly traverse opesiules^ to reach the 

 frontal membrane; or, in Steganoporella, to a pair of 

 depressor muscles and a pair of divaricators. The series 

 probably includes a large number of the Cretaceous forms 

 allied to Ony chocel la, as well as the recent 0. abyssicola, 

 Smitt, etc. Some of the recent species sometimes placed in 

 the Membraniporidae may also belong to this group. 



(4) The Cribrilinidag, including Membraniporella and 

 Cribriliua (part), are forms in which a calcareous frontal 

 wall is developed, ontogenetically, by the overarching of a 

 series of marginal calcareous spines. This process has been 

 observed by various authors in a certain number of species. 

 I have shown above that C. radiata differs in important 

 respects from the typical Cribrilinidas. 



(5) In Umbouula verrucosa the frontal shield originates 

 ontogenetically as a calcareous lamina, ]jerforated by a con- 

 spicuous series of marginal pores, which grows over the 

 Flustrine frontal membrane, leaving* a Avide space between 

 itself and that membrane. The parietal muscles develop in 

 the situations which they will occupy in the mature zooecium. 

 I consider that the marginal pores correspond with the 

 origins of as many Cribrilina-like spines which have 

 united to form the frontal shield. The external calcareous 



1 MacGillivray (1887), p. 207. 

 ' Cf. Hai-mer (1900), p. 22S. 

 3 Ibid., p. 230. 



