BRYOZOA. 253 



Prasopora lenticularis.l 



at intervals of about 3 mm., each a mm. or more in diameter. Surrounding these 

 the surface is depressed to a variable degree, and occupied by angular zooecial 

 apertures of comparatively large size and very few mesopores. The average diam- 

 eter of these zooecia is about 0.28 mm. On the rounded ridges between the depres- 

 sions the zooecial apertures are circular and smaller, averaging about 0.2 mm. in 

 diameter ; here they are also completely surrounded by a row of small mesopores. 



Internal characters: The tangential section figured on page 248 shows in the 

 upper half one of the maculae with the large zocecia surrounding it and occupying 

 the depressed hexagonal surface spaces. Between these zocecia the mesopores are 

 very few, but farther out, in spaces representing the ridges (lower third of figure), 

 the mesopores usually completely isolate the, here also smaller, zooecia from each 

 other. Acanthopores are wanting. 



In vertical sections (fig. 15c) the maculae appear as numerous, small, subequal, 

 closely tabulated tubes. One or two similar mesopores occur between many of the 

 zocecia in the inter-macular spaces. In the zooecial tubes the transverse partitions 

 are quite different. The appearance of the cystiphragms depends upon the direction 

 in which the section passes through them. When this is at right angles they appear 

 (see the central tube of the three shown in the figure) as narrow loops projecting 

 inward from the walls. Sometimes a complete diaphragm passes between each pair. 

 A variety of appearances, some of them shown in the figure, result when the section 

 passes through the cystiphragms at other than a right angle. 



This species is readily distinguished from its associate, P. insularis Ulrich, as well 

 as from all the other species of the genus known to me, by the division of the surface 

 into subhexagonal depressed spaces. The zoarium is also unusually thin, while the 

 internal structure is peculiar enough to be distinguished at once, even from its 

 nearest allies, P. affinis Foorcl, and P. selwyni Nicholson. Still, I am not fully satis- 

 fied that the form is in all cases to be distinguished specifically from P. affinis, small 

 specimens of which have recently been found associated with it. 



Formation and locality. Rather rare in the Galena shales at several localities in Goodhue county, 

 Minnesota. The types of the species are from the equivalent Trenton limestone at Ottawa, Canada. 



Mus. Reg. No. 7625. 



PRASOPOKA LENTICULARIS, n. sp. 



PLATE XVII, FIGS. 22-25. 



Zoarium small, lenticular, beginning its growth upon foreign bodies, as far as 

 observed, about 12 mm. in diameter and 1 mm. thick. Zocecial apertures regularly 

 arranged, oval, twelve or thirteen in 3 mm., each of the smaller or average size 

 0.18 by 0.23 mm. In the clustsrs a few of the largest miy attain a size of 0.33 by 



