A, ^ ,, , Myozoa. 225 



At actoporella crassa.] 



angles of junction and between the angles in the walls of both the zocecia and meso- 

 pores. In most cases the acanthopores are not very conspicuous at the surface. 

 This is in part di^ -to attrition, since on protected spots they are distinct enough^ 

 giving the surface a nainutely granulose character of somewhat coarser and looser 

 pattern than in A. typicalis. In a variety represented by a dozen or more specimens 

 these structures are much more strongly developed. Indeed, these specimens 

 remind one very much of A. schucherti Ulrich, so far known only from the upper beds 

 'Of the Cincinnati group in Ohio. 



Internal characters: These are so well illustrated on the two plates above cited 

 that detailed descriptions are rendered unnecessary. It will suffice to say that the 

 mesopores, though usually rather large, are variable in size and distribution, that 

 the zooecia in the spaces between the aggregations of large cells are often in contact 

 with each other for more than half their circumference, that the walls are a little 

 thicker than in A. typicalis, the acanthopores less numerous, larger, and yet not pro- 

 jecting inward so much, while the tabulation of both sets of tubes is much less 

 compact. 



As has been intimated, two varieties might be distinguised from the typical 

 and more common form of the species. The first differs in having larger zooecia, the 

 second in having much stronger acanthopores, these being larger and prominent 

 enough at the surface to obscure the view of the zocecial apertures. The latter may 

 be compared with A. crassa, the following species, but they are too distinct in verti- 

 cal sections to be confused. 



Formation and locality. — Middle third of the Trenton shales at Minneapolis, St. Paul, Fountain, and 

 other localities in Minnesota where that horizon is exposed. 



Mus. Reg. No. 5985. 



Atactoporella ceassa, n. sp. 



PLATE XV, FIGS. 18-21. 



Zoarium a small, irregularly hemispheric mass, growing upon some foreign object, 

 2 or 3 cm. in diameter and 5 to 10 mm. in hight. Surface without monticules, nor 

 have distinguishable clusters of either large or small cells been detected. Zocecial 

 apertures subangular, rounded or irregularly outlined, inclosed by thick walls, prom- 

 inently elevated at most of the angles of junction into strong acanthopore spines ; 

 about fourteen apertures in 3 mm. Mesopores less numerous than usual, small and 

 difficult to detect at the surface. 



Internal characters: Considerable variety of structure, depending upon age, is 

 exhibited in different tangential sections or in parts of the same section. These, as 



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