BRYOZOA. 577 



volutions in 2 cm. Shaft rather thin just above a flange, then 

 gradually thickening and slightly curved outward to the mar- 

 gin of the succeeding flange. The under side of the flange is dis- 

 tinctly marked by the branches in the typical form, but much 

 less distinctly in the variety mentioned. Fenestrated expansion 

 usually not over one cm. wide, forming an angle of about 65 

 with the axial line. Branches about 21 in one cm., rounded on 

 the reverse, with a moderate keel on the obverse, separating 

 two rows of small scarcely elevated zooecia apertures, of which 

 from 20 to 22 occur in five mm. Keel with sharp spines three 

 or four to each fenestrule. Fenestrules subquadrate, a little 

 longer than wide, 16 or 17 in one cm. Dissepiments thin, 

 scarcely half the thickness of the branches. In the large variety 

 the fenestrules are somewhat longer, there being only about 15 

 in one cm. 

 Diam. of shaft of strongest ex. 1.8 mm., No. of vol. in 2 cm., 7.0. 



smallest " 1.2 " " " 7.2. 



average " 1.5 " " 6.7. 



av. large var. 2.5 " " " " 5.0. 



Although quite distinct and nearly as abundant, it requires 

 care to distinguish this species from A. communis. These two 

 species agree closely in the number of volutions and size of the 

 axis. However, with a little experience the student will soon 

 learn to detect their individual peculiarities at a glance. Thus 

 in A. proutanus the flange is directed toward the distal extrem- 

 ity and furrowed on the lower side, while in A. communis the 

 flange and fenestrated expansion extends out from the shaft at 

 nearly a right angle. The form mentioned as a large variety 

 may prove a distinct species. The specimens approach A. in- 

 vaginatus in form and were at first supposed to belong to that 

 species, but as the characters of the fenestrated expansion 

 agree more closely with typical A. proutanus than A.invaginatus, 

 I have thought it best to refer them, provisionally, as above. 

 Position and locality: Common at Sloan's Valley, Ky. It 

 also occurs at Chester, 111., and other localities in the State. 



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