;,,,,„ . feRYOZoA. . 227 



Ataetoporella rahiosa.1 



a little larger, angular, with very thin walls, and while the mesopores are readily 

 distinguishable and one or two to each zooecium, the acanthopores are so small as 

 to be practically wanting. 



Internal characters: To obtain the fully matured characters of this species it 

 is necessary to make the tangential section as nearly superficial as possible. In 

 this outermost region the zocecial wells are of moderate thickness and infl.ected more 

 or less strongly at the points occupied by the acanthopores. The mesopores here 

 appear as mere irregular interstices between the rounded walls of the zocecia. At 

 deeper levels in the section the acanthopores become smaller and at last indetermin- 

 able, the zooecia prismatic and their walls thin, and the mesopores more distinct. 

 The appearances now are just as in transverse sections of the axial region. Vertical 

 sections show that cystiphragms and diaphragms are developed throughout the 

 tubes from their origin in the axial region to their superficial orifices. These struc- 

 tures are, however, a little more clbsely arranged near the surface of the zoarium 

 than in the axial region. Mesopores are likewise present in both regions, but these 

 seem gradually to expand as they bend out of the axial region and to assume the 

 characters of true zooecia. At the same time new mesopores are interpolated, but 

 these do not develop into zooecia. On the contrary the tendency is to close them up 

 by deposit on the zooecial walls. 



This is the second species of the genus in which the zoarium rises into the 

 ramose form. The first is the A. newportensis Ulrich, from the lower beds of the 

 Cincinnati group at Newport, Ky. The latter has larger monticules, more numerous 

 mesopores, less abudant acanthopores, and thinner zooecial walls. The ramose habit 

 of growth will distinguish it from the other species of the genus. Several species 

 externally similar, but internally widely different, occur in the same beds with 

 A. ramosa. One of these is the Callopora pulchella, var. persimilis, another is the 

 Homotrypa tuberculata of the present work, while a third is the Batostoma montuosum. 

 With a little practice these will be distinguished readily enough, even without the 

 aid of thin sections. When the surface is a little worn the student may find it diffi- 

 cult to separate the species from montiferous examples of Monticulipora arborea, a 

 species found chiefly in the overlying Galena shales. When both are in a good 

 state of preservation he will find that the walls of the Monticulipora are somewhat 

 thicker and much more minutely granulose, and that true mesopores are wanting. 



Formation and ZoeaH^^.— Upper third of the Trenton shales, near Cannon Tails, Minnesota, where it 

 is associated with an abundance of Prasopora conoidea and Phylloporina eortieosa. 



