POLYZOA OK, BRYOZOA. 427 



though sometimes quite irregular crusts, growing upon foreign 

 bodies, and often superimposed in successive layers one above 

 the other ; the cells being easily recognised by. their arched, 

 subtriangular, or crescentic mouths. 



The Secondary and Tertiary Polyzoa are so numerous and 

 varied, and from their small size are so difficult of study, 

 that it will be sufficient here to give an extremely brief and 

 general review of the leading groups. Taking the Cheilosto- 

 matous Polyzoa first, we find that only a certain number of 

 the known families have been as yet detected in the fossil 

 condition. The first of these is the family CelluJariada, in 

 which the cceuoecium is branched and erect, and the branches 

 are linear and composed of cells arranged in the same plane. 

 As the type of this family we may take the genus Scrupocd- 

 laria (fig. 269, E), of the Tertiary and Kecent periods. In 

 the family of the Salicornariadce the coenoecium is erect and 

 dichotomously divided, the branches being cylindrical, and 

 the cells disposed round an imaginary axis. In Salicornaria 

 (fig. 269, c), of the Tertiary and Eecent seas, the surface is 

 divided into rhomboidal or hexagonal spaces, representing 

 the front walls of the cells, and irregularly-disposed avicu- 

 laria are present. In the family of the Hippothoidce the poly- 

 zoary is creeping and attached to foreign bodies, the cells 

 being pyriform, distant, and arranged in linear series. The 

 principal genus is Hippothoa (fig. 268, c), which is mostly 

 Tertiary and Recent, but which, as we have seen, is repre- 

 sented by a well-marked form as early as the Lower Silurian. 

 More important than any of the preceding is the family of 

 the Membraniporidce, in which there is an encrusting cal- 

 careous or corneo-calcareous polyzoary, composed of horizontal 

 and contiguous cells. In the comprehensive genus Membrani- 

 pora (fig. 261) the front wall of the cell is to a larger or 

 smaller extent membranous, so that in all but actually fresh 

 specimens it appears to be to a corresponding degree deficient 

 and open, while the cells are surrounded by an elevated 

 border. Numerous species of the genus are found in the 

 Cretaceous, Tertiary, and Eecent periods ; and some ancient 

 types (such as the previously-noticed Paleschara of the Si- 

 lurian) appear to be nearly related to this. An equally 



