THE PALEONTOLOGY OF MINNESOTA. 



[Pachydlctya. 



over the whole surface. There seems to have been a cessation of developement in 

 some places, causing the formation of irregular furrows, in which the old zooecial 

 apertures are partly closed by a sheet of dense material. Thin sections failed to reveal 

 anything unusual, hence, we may safely assume that these specimens present merely 

 an abnormal condition of the species. 



Of internal peculiarities brought out by tangential sections the most striking 

 are, (1) the unusual brevity of the end spaces. In many cases these are so short that 

 the outer lines of the ring-like walls of succeeding zoo3cia are often nearly in con- 

 tact. Generally the length of these spaces is less or about equals half the transverse 

 diameter of the zouecia ; (2) the continuous longitudinal lines of median pores (there 

 is as a rule only one in each interspace between the rows of zooecia) appears more 

 flexuous than usual ; and (3) the maculae or solid spots, which do not interrupt the 

 course of the lines of median tnbuli. A number of isolated tubuli, otherwise seem- 

 ingly of the same nature, occur between the lines mentioned. 



In. vertical sections the zooecial tubes frequently have diaphragms, their course 

 to the surface is less direct than common, and the interspaces or walls unusually 

 thin. 



The growth and maculose surface distinguishes this species from the other Min- 

 nesota forms of the genus, none of which are found, however, in the same beds with 

 P. occidentalis. Though perhaps still to be regarded as intermediate in some respects 

 between P. acuta Hall, sp., and P. fenestelliformis Nicholson, sp,, further investigation 

 proves the relationship to those species to be more remote than I thought at first. 

 It seems also to have preceded both in time. Compared with the first it is found to 

 differ in its mode of growth, the zoarium being wider, in the character of the inter- 

 spaces, and in the maculae which are wanting in that species. The second has larger 

 zorecia, and both present well marked internal differences. 



Formation and locality. Rather abundant in the upper third of the Trenton shales at St. Paul, 

 Minnesota. A few specimens also from the same horizon in Goodhue county. 



Mus. Beg. Nos. 5949, 7646. 



Section b: Species in which the width of the zoarium is limited, and the margins 

 subparallel. 



PACHYDICTYA FIMBRIATA Ulrich. 



PLATE VIII. FIGS. 28-34; PLATE IX, FIGS. 13 and 14. 



Pachydictya fimbriata ULRICH, 1886. Fourteenth Ann. Rep. Geol. Nat. Hist. Sur. Minn., p. 75. 



Zoarium rather small, ramose, the branches with subparallel margins, from 2 to 

 5 mm. wide, averaging a little over 3 mm., thin, the thickness rarely exceeding 0.5 

 mm.; bifurcations dichotomous, occuring at variable though generally at long inter- 



