300 PALAEONTOLOGY OF ILLINOIS. 



rinse," and especially in the footstalk. The discovery of that 

 structure in such unequivocal Bryozoa as the FENESTELLHXE is 

 of the greatest importance in its bearing upon the question of 

 the zoological position of the FISTULIPORULE. In the proper 

 place we will consider it in greater detail. 



IV. Acanthopores. A large number of Palaeozoic Bryozoa, 

 especially among the TREPOSTOMATA, present, when in a good 

 state of preservation, a greater or less number of blunt spine- 

 like structures, which are placed around the zocecia apertures, 

 usually at the angles of junction between adjoining cells Some- 

 times they also occur at some point between the angles, while 

 in other forms, notably such as Leioclema species, in which the 

 mesopores are very numerous, they are apparently included 

 within the substance of the walls of the zocecia. These peculiar 

 appendages present themselves at the surface as so many blunt 

 projections, varying considerably in prominence and size in 

 different species, In the genus Dekayia, they attain a greater 

 size than in any other, being in some species almost or quite 

 as large as an average zooecium. Ordinarily they are much 

 smaller than this, while in some forms they are so much re- 

 duced in size that it is difficult to distinguish them from the 

 elevated surface terminations of the "median tubuli" (e. g. 

 Rhombopora species, and Leioclema? araneum). Thin sections 

 show conclusively that they were all originally hollow, and the 

 apical aperture of the spines may in rare instance be detected. 

 Generally, however, the summit of the spines appears to be solid, 

 but this fact may be easily accounted for by the readiness with 

 which the excessively minute apertures would be filled up by the 

 matrix or by infiltrated calcite. As stated, sections prove beyond 

 a doubt that we are not dealing with mere superficial ornaments, 

 but that they extend into the substance of the peripheral region 

 of the zoarium, in which they can be traced to a depth equal 

 to that reached by the "mesopores". As seen in tangential 

 sections they appear as circular spots of somewhat darker or 

 lighter shade than the wall substance immediately surrounding 

 them. 



