156 THE PALEONTOLOGY OF MINNESOTA. 



[Pachydctya. 



specimen, which may be assumed to represent the typical form of the species, the 

 zoo3Cial apertures are elliptical, rather small in the central rows where, however, 

 they are separated by comparatively long intervals, with twelve in 5 mm. In the 

 marginal row the zooecial apertures are rather oblique and conspicuously larger, 

 and here only eight or nine are to be counted in the same space. The non-porifer- 

 ous margins are wide, and where the preservation is good, have the usual oblique 

 granulose striation. The interspaces form faint longitudinal ridges, while a delicate 

 rim is to be detected here and there around the sunken apertures. In some of the 

 other fragments the surface is preserved better, or, as is more likely, it presents a 

 less aged condition, and in these the peristome is more distinct, as is also a thin 

 raised line passing between the central longitudinal rows of apertures. The general 

 eftect, therefore, is much as in fig. 32 of the same plate, only the zooecial apertures 

 are narrower and farther apart, and the marginal ones larger. 



The New York, Canadian and Vermont specimens, or as we may call them, the 

 eastern form of the species, is fairly constant in every respect. The zoarium divides 

 dichotomously at rather long intervals, the length of these varying between the 

 extremes of 10 and 20 mm., while the width of the branches between the bifurcations, 

 where the margins are parallel, is rarely more than 3.0 mm., and so far as noticed, 

 never less than 2.5 mm. The number of rows of zocecia is generally seven or eight. 



In the western form, however, we find a greater or less degree of instability in 

 nearly* every character. This is to be remarked especially of the Minnesota speci- 

 mens. The branches, as a rule, are considerably wider, the average varying between 

 3.5 mm. and 5.5 mm. Still, it is not rare to find specimens, particularly among those 

 from the lower beds of the Galena limestone, that are narrower, with the average at 

 about 2.0 mm. Figure 16 represents an example that may be compared with the 

 eastern form in the matter of branching, but in a great majority of the western 

 specimens the divisions are much closer, the average distance between them being 

 about 10 mm., and in many less. Another point to be noted is the tendency to 

 irregularity in the growth of the zoarium of the western form, abortive branches, 

 trifurcations and unparallel margins being common, while its appearance in general 

 is less rigid than is prevailingly the case in the eastern form. The non-poriferous 

 margin may be wide or narrow, but it is rare, if it ever occurs, to find an amount of 

 difference in the size of the zooecial apertures in the marginal and central rows 

 equalling that prevailing in the eastern form. As a rule, the difference may be 

 stated to be greatest in the smaller examples and least in the wide ones. The num- 

 ber of zooecia rows varies from six to eighteen, with eleven, twelve and thirteen the 

 number most frequently met with . In the central rows twelve, thirteen or fourteen 

 apertures, the two last numbers more common than the first, occur in 5 mm. In 



