BRYOZOA 







ATACTOPOREIXA TYPICALIS. var PRJICIPTA, m. mr. 



PLATE XV. FWS. IB and V: PLATE XVIII. FIGS. 1-4. 



This form, though much earlier. is too much like the Cincinnati A. iypienlia 

 L'lrich (Joar. Cin. So:. Nat. Hist., vol. vi. p. 24-S. 1S S 3>, to be distinguished specifically. 

 The resemblance is so close that the original description may, with a few trifling 

 alterations, be made to tit the Minnesota form. Hundreds of feet of strata, however, 

 intervene between the two horizons respectively held by the two varieties, in which 

 the species is unknown. It would seem, therefore, to be a case of reapparition. not 

 necessarily of the same species, but of the same type of structure, similar to the cases 

 of Callopora and Dekayella noticed in this work. 



The characters of var pnecipta are as follow 



Zoarium forming small thin crusts, rarely exceeding 1 mm. in thickness, over 

 ramose Bryozoa and shells. Surface minutely spinulose. without monticules, except 

 in rare instances (see plate XVIII. tig. 4). but exhibiting at intervals of about 2.5 mm., 

 measuring from center to center, clusters of cells slightly larger than the average 

 between which the interspaces are also a little thicker than usual. Zooecial apertures 

 floriform, the walls thin and at each inflection raised into a small spine, the surface 

 extension of an acanthopore, arranged in moderately regular, diagonally intersecting 

 series, averaging fourteen in 3 mm. Interspaces narrowing with age, very thin, with 

 the zo-ecial walls largely in contact, the apertures direct and the mesopores small 

 and easily overlooked in fully matured examples : thicker, with the mesopores more 

 distinct and the zooecial apertures drawn out obliquely in younger stages of growth. 

 Acanthopores numerous, small bat sharply elevated, situated in the zocecial walls, 

 four to seven, usually five or six. around each aperture. 



Interifil >:harn,den: In tangential sections the zo<:ecial walls are very thin and 

 indented more or less sharply at from four to seven points in their circumference 

 These inflections of the wall are emphasized by the acanthopores. one of which 

 occurs at each point and appearing in nearly all cases to be formed on the inner 

 side of the wall. A few of the zooecia may be completely isolated by the interven- 

 tion of irregularly-shaped meaopores. but as a rule they are in contact at limited 

 points. The mesopores never form more than a single row, and their walls are 

 entirely without acanthof The crescentic cut _ : the cystiphragms, some- 



times two or even three in each, are to be seen in each zo<:ecium. On account of the 

 nearly equal thickness of these edges and of the walls of the zooeda and mesopore- 

 is often difficult to discriminate between the varior. -hown in tangential sec- 



tio: od vertical sections are difficult to prepare, because of the tenuity of the 



