296 PALEONTOLOGY OF ILLINOIS. 



7. The frondescent or palmate zoarium is intimately con- 

 nected with the ramose, the arrangement of the zocecia being 

 the same. It usually consists of leaf-like expansions with the 

 edges rounded and celluliferous. Good examples are Heterotrypa 

 frondosa D'Orbigny, sp., and Homotrypa dawsoni Nicholson. 



8. The segmented zoarium is characteristic of a number of 

 Palaeozoic Bryozoa. In these the complete zoarium is com- 

 posed of more or less numerous, simple or branched segments, 

 with each end slightly bulbous and subsolid. This type of 

 growth is seen in ArthrostyJus, Arthropora, Arthroclema, Dicra- 

 nopora and Helopora, all Silurian genera. It has also been 

 observed in a new form from the Hamilton group of New York. 



The species of Ptilodictya may also be included with the seg- 

 mented Bryozoa, since the basal extremity of the zoarium in 

 this genus is pointed and was inserted in the socket-like depres- 

 sion of an attached basal expansion. 



9. The fenestrated zoarium, in which the colony is composed 

 of slender branches, celluliferous on one side, which anastomose 

 with each other as in Phyllopora and Phylloporina, or are 

 united at regular intervals by non-poriferous processes (dis- 

 sepiments) the whole forming a flabellate, spiral or infundibular 

 expansion. Fenestella, Polypora and Semicoscinium are ex- 

 amples. 



Archimedes and Lyropora are modifications of this type of 

 growth. In the first there is a solid spiral axis; in the second 

 two strong diverging supports, between which the reticulate ex- 

 pansion is spread. 



10. The pinnate zoarium differs from the preceding in being 

 composed of a straight central branch and more slender lateral 

 branches, which spring from the sides of the main stem. Acan- 

 thocladia and Pinnatopora. 



11. Surface characters. Aside from the superficial characters 

 of the zocecia, which will be treated in another place, the sur- 

 face of a large number of Palaeozoic Bryozoa presents certain 

 peculiarities, which, on account of their conspicuousness, demand 

 special consideration. Foremost among these are the structures 

 commonly known as "monticules" and "maculae" which, though 

 properly enough applied to designate distinct conditions, are 



