302 PALAEONTOLOGY OF ILLINOIS. 



ones, characterizing the genera Batostomella, Amplexopora, 

 Atactopora and Rhombopora, were most probably occupied by 

 somewhat different yet homologous appendages. These may 

 find an explanation in the stout, chitinous or calcareous bristles 

 which are found upon the walls of the zooecia in many CHEILO- 

 STOMATA. After the death of the zooid these bristles fall off, 

 leaving minute pits in the summits of small blunt spines. 



True acanthopores are also present in a large proportion of 

 the FENESTELLID^E and ACANTHOCLADIID^E. In Fenestella, Ptylo- 

 pora and Septopora, the mesial carina very often bears strong 

 spines, which sections prove to have been hollow and con- 

 structed upon the same general plan as the "acanthopores" of 

 the TREPOSTOMATA. 



In Polypora and other genera having more than two rows of 

 zooecia they are distributed among the cells jWith greater or 

 less regularity. 



The apical apertures are not often preserved, but they are 

 shown in a beautiful manner in specimens of several species de- 

 scribed in this volume (Fenestella wortheni, PI. 52, fig. 5, 5a, 

 and Ptylopora cylindracea, PI. 66, fig. 2). While I regard these 

 "acanthopores" as probably having been occupied by vibracular 

 structures, some other explanation seems necessary to account 

 for the peculiar "dimorphic pores" occurring so abundantly upon 

 the non-poriferous side of the branches in Septopora, Acantho- 

 cladia fruticosa and Semicoscinium infraporosum. These pores 

 are also present on the celluliferous side of several species, where 

 they are scattered among the zooecia. In the species last men- 

 tioned, we find them on each side of the prominent mesial keel. 

 These cannot have contained the same kind of modified zooid 

 as the acanthopores, since they are generally much larger than 

 the latter, and because in most cases (notably Septopora 

 biserialis Swallow) both kinds of pores occur in the same 

 species. If in the "acanthopores" we see the supports of vibracu- 

 lar structures, would it be unreasonable to suppose that the 

 "dimorphic pores" were the receptacles of "avicularia"? I see 

 nothing in their structure to negative the supposition, nor 

 would these be the only instances in which both avicularia and 

 vibracula occur together, as, according to Hincks, both kinds 

 of appendanges are present in two genera of the CHEILOSTOMATA. 



