1915 6 F. A. Bather — Studies in Edrioasteroidea. 



in publication does not seem advisable. Various reasons make it 

 convenient to deal with the species in the order of their geological 

 age, beginning with the oldest, which is the most complete and is 

 taken as the type-species of the genus. The new species are — 



Pyrgocystis sardesoni, Lower Ordovician, Minnesota (genotype). 

 Pyrgocystis grayae, Upper Ordovician, Girvan. 

 Pyrgocystis ansticei, Middle Silurian, Shropshire. 



To this genus are also referred some fossils from the Middle 

 Silurian of Gotland which C. W. S. Aurivillius in 1892 regarded as 

 representing seven species of the Cirripede genus Scalpellum. 

 Though possibly to be reduced to only two species, they all appear to 

 be distinct from the new species mentioned above. 



The essential characters of the genus are displayed most clearly by 

 the genotype P. sardesoni, to the description of which we now proceed. 



A. Pyrgocystis sardesoni n.sp. (Plate II, Figs. 1-6.) 



In March, 1901, Dr. F. W. Sardeson, of the University of 

 Minnesota, himself a student of Palaeozoic Pelmatozoa, was so 

 generous as to send to me for study and description the rare and 

 interesting fossils which give rise to this new genus, although he had 

 originally intended to write on them himself. Not only this, but he 

 has permitted me to retain the best specimen, returning him the 

 others. Preliminary studies were made and drawings were prepared 

 without undue delay, but various causes have up till now prevented 

 the publication of the paper. This may seem a poor return for 

 Dr. Sardeson's confidence, but the work at any rate has not suffered 

 by keeping. It only remains for me to express my profound appre- 

 ciation of his extraordinary kindness. 



Horizon and Locality. — " The three specimens," says Dr. Sardeson, 

 "compose all of the set, collected from the Ordovician Galena series, 

 Stictopora bed ; at St. Paul, Minnesota. The exact spot is a quarry 

 on the west bluff of the Mississippi river, opposite the outlet at the 

 east end of ' Pickerel Lake as given on maps of St. Paul city." 



The position of the Stictopora or, more correctly, Stictoporella bed 

 in the Minnesota section is well known, but opinion as to the 

 correlation with other regions has undergone much change The bed 

 lies above the Platteville limestone and at the base of a series of 

 shales successively termed Trenton, Black River, and Decorah, and 

 now considered by E. 0. Ulrich to represent the upper part of the 

 Black River formation (see his " Revision of the Palaeozoic Systems", 

 Bull. Geol. Soc. Amer., vol. xxii, pi. 27; August, 1911). The 

 Stictoporella bed comprises soft green shales and thin-bedded lime- 

 stones, and is particularly rich in Polyzoa. According to R. S. Bassler 

 (December, 1911, Bull. U.S. Nat. Mus., lxxvii, p. 38), the Decorah 

 Shales correspond generally to the beds of Baltic Russia from the 

 "Wassalem beds (D 3) down to the Glauconite sandstone. But since 

 the Kimmswick Limestone of Missouri (which corresponds to the top 

 of the Decorah shales or even overlies them) contains a species of 

 Echino splicer a said to be scarcely, if at all, distinguishable from 



