362 PALAEONTOLOGY OF ILLINOIS. 



the margins of the primary branches new ones are given off at 

 the same angle as the secondary branchlets, and these again 

 are provided with branchlets the same as the original stem 

 (see PL LXVI, figs. 7 and 8). 



From the above it will be seen that the ACANTHOCLADIID.E and 

 FENESTELLHLE are distinguished by a zoarial difference, but this 

 difference is merely the result of some peculiarity in the de- 

 velopment of the zooecia, causing celluliferous branchlets to 

 take the place of solid dissepiments. In Ptilopora McCoy, 

 which might with equal propriety be arranged with the FENE- 

 STELLID.E, we have intermediate conditions. It is a type that 

 commenced in early Devonian times with species of FenesteUa, 

 in which some of the branches were disproportionately thick- 

 ened, and the thinner branches at the same time arranged in 

 an indistinct pinnate manner. The type cannot rank as a sep- 

 arate family, and the ultimate position of the genus depends 

 largely upon the relative importance attached to the presence 

 of dissepiments on the one hand and the pinnate arrangement 

 of the zoarium on the other. (See remarks on the genus fur- 

 ther on). 



(6) SPHRAGIOPORID.,E: This family is proposed for the re- 

 ception of a single genus, of which, so far as I have seen, only 

 one or possibly two species are known. This is, however, so 

 distinct from all the previously established families, that I feel 

 justified in adopting the course here pursued. For particulars 

 relating to the structure of this unique bryozoan, the reader is 

 referred to the specific description further on. 



(7) PHYLLOPORINHXE: The genera comprised in this family 

 have been established upon species usually referred, heretofore, 

 to the FENESTELLID^E. A minute comparison of their interiors 

 having brought to light certain peculiarities in zooecial struc- 

 ture, a new family seems to be demanded for them. These dis- 

 tinctive features have already been pointed out in my general 

 remarks on classification, and to prevent repetition, the reader 

 is referred there, and to the descriptions in the subjoined synop- 

 sis of classification. 



(8) ARTHROSTYLID.E. This interesting family of Bryozoa, 

 especially in its typical members, reminds one of the recent 

 CELLARID.E, yet beyond the articulated zoarium and a general 



