THE PALEONTOLOGY OF MINNESOTA. 



[Pachydictya. 



PACHYDICTYA ELEGANS. n. sp. 



PLATE VIII, FIGS. 18 and 19. PLATE IX, FIGS. 8 and 9. 



The nearly complete type specimen began its growth on the extremity of some 

 undetermined ramose bryozoan. The basal expansion is small, and its surface 

 largely Covered with granulose striae. At its edges, where it grew downward on the 

 foreign body, a few apparently normal zooecia were developed. From the exceed- 

 ingly short, neck-like constriction above the base, the erect portion of the zoarium 

 divides at once into three branches, and two of these continue to divide dichoto- 

 mously with extraordinary frequency, the average distance between bifurcations 

 being only 5 or 6 mm. This frequent division caused the zoarium to spread with 

 unusual rapidity ; some of the inner branches must have overlapped if continued. 

 We may assume, however, that with age, beyond that shown in this example, the 

 outer or subsequent divisions became less frequent, or at any rate, dependent upon 

 the space available for lateral development. Branches 2.5 mm. to 5.0 mm. wide, 

 thin, edges sharp, non-poriferous, border wide, obliquely grano-striate. In the 

 thickest specimens the celluliferous portion of the branch rises abruptly from the 

 wide non-poriferous borders, the growth of the latter having failed to keep pace 

 with that of the zooecia. Under a good hand lens the surface presents a highly 

 ornamental appearance, the arrangement of the zooecia and sculpture of the 

 interspaces being very regular. Apertures elliptical, separated from each other 

 by spaces as wide as their shorter or transverse diameter. In the central rows 

 the arrangement is alternate, with thirteen or fourteen in 5 mm., measuring 

 lengthwise, and seven of the central rows in 2 mm., transversely. Those in the 

 marginal rows slightly oblique, a little larger than the average and separated 

 by correspondingly wider interspaces, so that a smaller number occurs here in a 

 given space than in the central series. Surrounding each aperture a sharply defined 

 rim or peristome, and rising from the center of the depressed spaces between the 

 longitudinal rows, a faintly flexuous, thread-like line. On the best preserved por- 

 tions of the surface, both the longitudinal lines and the peristomes are seen to carry 

 a row of minute papillae. Over the central part of the surface the depressed end 

 spaces are narrow and usually empty, but toward the margins, where they are 

 wider, they are occupied by a gradually increasing uumber of papillae, at first 

 isolated, then forming short outwardly tending rows. 



Provisionally I propose to place here a number of specimens agreeing in all 

 respects with the type of the species, save in this, that they bifurcate at less frequent 

 intervals. The interspaces in many are a trifle thicker, but as these specimens are 

 heavier and evidently older, that is to be expected. 



