American Paloeozoic Bryozoa. 



127 



Jiliasa, D'Orb., is one of these. I have specimens of this species in 

 which no less than twent} r successive "immature" and "mature" 

 zones may be counted. The "immature" zones are marked by very 

 thin tube-walls, and remote diaphragms, i, e., from one to two tube- 

 diameters apart; while in the "mature" zones the walls are slightly 

 thickened, and the diaphragms crowded. 



IV. Interstitial Cells and Tubes. — Though these are present in 

 greater or less number, in the most of the Monti culiporidaz, they are 

 never so numerous as in the Fistuliporidce, In the Monticuliporidce 

 the interstitial tubes always have distinct walls, and more numerous 

 diaphragms than the true tubes. The diaphragms too are always 

 complete and approximately straight. In the Fistuliporidce the inter- 

 stitial cells are not produced into tubes as in the Monticuliporidce, 

 and the cells of each of the various interstitial cell-layers are quite in- 

 dependent of those of the preceding layer, inasmuch as they are not 

 placed directly over each other, but indiscriminately fill the spaces inter- 

 vening between the true cells ; this produces a characteristic feature of 

 the Fistuliporidce, i.e., the vesicular interstitial tissue, which is always 

 a conspicuous feature in a longitudinal section. I am not certain that 

 the minute tubes forming the " macular" of some species, as for in- 

 stance, Atactopora maculata, Ulrich, are reall}* of the same nature as 

 the usual interstitial tubes. They are, however, identical in structure 

 with the tubuli forming the " maculae" of some of the Ceramoporidce 

 (e.y., the species of Crepipora, Ulrich). 



V. Spiniform Tubuli — A majority of the species of the Monticulip- 

 oridce, and some of the Fistuliporidce, present, when well preserved, a 

 greater or less number of blunt, spine-like structures, which are placed 

 at the angles of junction of the cells, or on the line separating adjoin 

 ing cells ; or, as is not infrequentl}- the case, they are included within 

 the substance of the walls of the cells. From what is now known of 

 these structures, there seems to be little room to doubt, that in almost 

 all cases, they were primarily hoUow, though it is very rare that an 

 aperture may be detected at the summits of these spines. This fact 

 may be easily accounted for by the excessive minuteness of these 

 apertures, and the readiness with which they would be obscured by 

 the infiltration of calcite. From sections we learn that the " spiniform 

 tubuli" extend into the substance of the zoarium to a depth equal to 

 that of the interstitial tubes, and "matura" portion of the zoarium. 

 They usually exhibit in their centers either a dark or clear circular 

 spot, which is surrounded by a concentrically laminated sclerenchyma. 



