i)r. F. A. Bather — Notes on Yunnan Cystidea. 75 



Silurian Fossils" (1915, Bull. 92 U.S. National Mus.), under 

 Holocystites. From the Bacine Limestone of Wisconsin and Illinois 

 there are the seven species already mentioned as described in 

 1864-5, as well as S. jolietensis Miller (1882), from Joliet, 111. 

 From the underlying Laurel Limestone near "Waldron, Ind., is 

 H. pustulosus Miller (1878). The Osgood Limestone of Indiana, 

 which is slightly older, Las yielded specimens in Jefferson county, 

 mostly from Big Creek, near Dupont, but some from the neighbour- 

 hood of Madison, and in Bipley county, mostly from near Osgood. 

 On these Miller based forty species. 



In time, then, these species are confined to a period extending 

 from the top of the Clinton to the middle of the Lockport, 

 approximately equivalent to the age of the Woolhope Limestone and 

 the lower half of the Wenlock beds. In space they are confined to 

 the south-eastern quarter of Indiana and a strip bordering Lake 

 Michigan on the south-west. That is to say, they inhabited the 

 Indiana Basin of the Mississippian Sea, which derived its fauna, 

 according to Schuchert (1910), from the Atlantic by way of the 

 Gulf of Mexico. The bare record of JT. glohosus, from the Rochester 

 Shale of Hamilton, Ont., 1 is for the present unsupported by published 

 evidence, but it was just on the border of the same Basin. 



2. Specific Characters in Megacystis. 



Everyone who has alluded to the forty-one species proposed by 

 S. A. Miller (especially P. H. Carpenter, 1891; Jaekel, 1899; 

 B. B. Bowley, 1903) has considered that they might be reduced to 

 a far smaller number. Even Miller himself wrote in 1892: "The 

 Holocystites are so variable in all respects that it is hard to tell 

 exactly what should be considered specific characters and what 

 should be regarded as variations among fossils belonging to the same 

 species." 



Here we obtain some guidance from the specimens of Sinocystis, 

 which no one could regard as representing more than three species. 

 The size and form of the theca are seen to vary within wide limits, 

 in part no doubt with age, and the absolute number of plates is 

 correlated with the growth of the theca. The positions of the 

 thecal openings are also subject to slight individual variation, 

 possibly connected with the position assumed by the theca. The 

 size and shape of the base depend, as usual, on the surface to which 

 the theca chanced to become attached, and on similar external 

 conditions. There is no doubt a difference of general form between 

 the pear-shaped S. loczyi, the more globose S. yunnanensis, and the 

 egg-shaped S. mansuyi, but this of itself would not enable one to 

 determine the species of such a specimen as I, 10. Slight differences 

 seem to persist in the form of the facets and cover-plates, and some 

 definite character might be deduced from a large series. But the 

 most easily recognized and quite constant differentia lies in the 

 individual thecal plates ; these differ in tumidity, but above all in 

 the size, form, and distribution of the diplopores. 



Returning to the Holocystites, we find that the pores are just the 



1 W. A. Parks, 1913, Canada Geol. Surv. Guide-book No. 4, p. 132. 



