BRYOZOA. 339 



(2) HETEROTRYPID.E: The zoarium in this family is fronde- 

 scent, ramose or iiicnisting. Usually the zooecia are polygonal, 

 more or less contiguous, with comparatively thin walls. In 

 other cases rounded or oval. The mesopores, though always 

 present, vary greatly in number, being exceedingly few in some 



(Dekayia). and quite numerous in others (species of De- 

 kayella.) Acanthopores are always present, sometimes being 

 of very large size (Dekayia). Generally they are small and 

 placed at the angles of junction between the zooecia. Their 

 cavity, too, is usually very small, but in species of Batostoma 

 it is larger than in any other member of the TREPOSTOMATA. 

 No cystiphragms are present, and the diaphragms are always 

 horizontal and conspicuously more numerous in the meso- 

 pores than in the zooecial tubes. Another peculiarity fre- 

 quently met with, especially in species of the typical genera 

 of the family, is the closing of the cell apertures on limited or 

 extended portions of the surface by a thin, apparently imper- 

 forate calcareous pellicle. 



The four genera, Heteroti'ypa Nicholson, Dekayia Ed. & H., 

 Dekayella. and Petigopora constitute the family as here under- 

 stood. These genera clearly have natural affinities, differing 

 from each other only in the number of mesopores, thickness of 

 zooecial walls, and in the form of the zoarium. Thus Hetero- 

 trypa is characterized by moderately thick-walled zooecia, com- 

 paratively few mesopores, and a frondescent or compressed 

 habit of growth; Dekayia by its thin walled zooecia, a ramose 

 or subramose zoarium, and by the practical absence of meso- 

 pores: Dekayella by the numerous mesopores, and two sizes of 

 acauthopores: and Petigopora by the parasitic habit of growth 

 and nou-celluliferous epithecal margin. 



(3) CALLOPORID.E: The zoarium in this family may be dend- 

 roid, subfrondescent, discoid, or, rarely, sub-pyriform. The 

 zo<Hda are sometimes polygonal, bu.t usually sub-circular, and 

 separated from each other by numerous angular mesopores, 

 which, when the zooecia are rounded, more or less completely 

 isolate them. Zooecial walls generally thin, rarely thickened 

 and ring-like in transverse section. Cystiphragms are wanting, 



