28 NEW YORK STATE MUSEUM 



mens were obtained by the writer in the very, low horizon of 

 Trenton limestone at Glens Falls which has been termed the Glens 

 Falls limestone and is below the base of the outcropping Trenton at 

 Trenton Falls. The species is there associated with the algae 

 Callithamnopsis delicatula and Coremato- 

 cladus densa 1 and other peculiar fossils. Billings 2 described 

 four species of Pleurocystites from the Trenton limestone at 

 Ottawa and one from the lower part of the Trenton limestone at 

 Montreal. Jaekel in his monumental Stammesgeschichte der PH- 

 matosoen has reduced the five species to two and one variety. 



While Billings saw the principal differential characters of his 

 species in the sculpturing of the plates and the form of the pectin- 

 ated rhombs, Jaekel, for phylogenetic reasons, lays the main 

 emphasis on the relative size of the plates of the anal side. By a 

 combination of these criteria he distinguishes the groups of (i) 

 Pleurocystites filitextus Billings, with P . e 1 e g a n s 

 Billings as a synonym, and the exornata Billings as a variety, 

 and (2) P . squamosus Billings with P . robustus as a 

 synonym and the variety anticostiensis Billings (from the 

 Richmond beds of Anticosti). 



P. filitextus is defined by this author as having radially 

 striated thecal plates and the upper pectinated rhombs long ; rather 

 large anal plates (about seventy) and stem joints with relatively 

 smooth collars. 



P. squamosus has the thecal plates almost wholly sculp- 

 tured by concentric lines ; the anal plates small (about five hun- 

 dred) ; the pectinated rhombs short and the stem joints longi- 

 tudinally striated. 



Applying these criteria to our form, of which we have seven 

 specimens, we find that it has prevailingly the character of P . 

 squamosus, that is, the small anal plates, the longitudinally 

 striated stem joints and the thecal plates, mostly marked by con- 

 centric lines. Alongside of these features our type exhibits, how- 

 ever, also in two specimens very well-developed radial striae, and 

 above all the pectinated rhombs are long and narrow throughout. 

 This form would, hence, to some extent, appear to combine the 



1 Ruedemann. Some Marine Algae from the Trenton Limestone of New 

 York. 'N. Y. State Mus. Bui. 133, p. 194. 1909. 



2 Billings. On the Cystidae of the Lower Silurian Rocks of Canada. 

 Ceol. Surv. of Canada. ' Fig. & Descr. of Canadian Org. Remains, decade 

 III, p. 46H. 1858. 



