INTEODUCTIOtf. 33 



which cancellated interzocecial tissue is present; but so far, I cannot 

 verify the occurrence of any species of this group in the Jurassic. 

 The differences between these three groups seem to me more im- 

 portant than variations in zoarial form, for they involve differences 

 in zooecial structure. I therefore propose to call them the Tubu- 

 lata, the Dactylethrata, and the Cancellata. 



The subdivisions of these orders must depend on zoarial 

 characters. It is natural to commence with the largest of the 

 three, which is also the most difficult, because the most simple. 

 The most primitive form is a simple linear zoarium Stomatopora. 

 This serves as the starting-point of several different lines of de- 

 velopment. On the one hand, the zocecia grow into ribbon-shaped 

 bands ; these expand into sheets, which may remain encrusting or 

 rise as fronds. For this group we may adopt the name Tubuli- 

 poridae. 



A second family is formed by the primitive zocecion giving rise 

 to banded zoaria in which the apertures are arranged in regular 

 transverse rows, instead of having them scattered irregularly. The 

 adnate bands give rise to erect forms, in which the zooecia only 

 open on one side. These form the family Idmoniidae. Those 

 Bryozoa which arise from a similar embryonic zooecion, but give 

 rise to solid bundles, instead of to sheets or unilateral dendroid 

 forms, represent a third group, for which we may accept Pergens' 

 name Entalophoridae. 



The second group of Tubulata arises from a different larval form, 

 which was originally described as the genus Pelagia or Defrancia. 

 The young stage of the zoarium is cupuliform, instead of stomato- 

 poriform. The zoaria may be discoid, as in Actinopora, or the 

 zocecia may be elongated into such types as Apsendesia and Fasci- 

 culipora. 



The Dactylethrata is a much smaller group, and contains the 

 three types, Reticulipora, Multiclausa, and Terebellaria. 



The Cancellata do not appear until the Cretaceous. 



The following summary of the classification includes the Jurassic 

 families, while a few of the principal families found in later 

 periods are mentioned, in order to suggest the lines of development 

 subsequently followed. The Cretaceous genera seem to fall easily 

 into places in this scheme. 



