THE DIATOMACE.E OF PHILADELPHIA AND VICINITY 21 



COSCINODISCUS EHR. (1838) 

 (coscinon, a sieve, and discus) 



Frustules solitary, cylindrical, compressed; valve circular or elliptical; surface flat or 

 sometimes convex near the border; markings more or less angular, radiating, sometimes 

 fasciculate; border usually well denned. Central space, if present, hyaline, sometimes sur- 

 rounded with a rosette of large cells. 



Chromatophores round, angular or irregular discs usually without pyrenoids (Karsten). 



Rattray's classification is here followed, so far as it refers to our species. 



Excentrici. Valves circular; central space absent; markings angular, in oblique, decus- 

 sating rows. 



Lineati. Central space absent; markings angular, oblique decussating rows straight. 



Fasdculati. Markings fasciculate, or sometimes only near the border. 



Radiati. Markings rounded or angular, more or less radiate. 



Elaborati. Valves elliptical, markings rounded. 



EXCENTRICI 

 COSCINODISCUS EXCENTRICUS EHR. 



Valve with a hyaline excentric space from which proceed, usually in six directions, 

 rows of polygonal markings decreasing toward the narrow, coarsely striated border, the rows 

 appearing convex toward the centre. Apiculi at unequal distances apart. Quite variable 

 in size. 



Common in the blue clay and along the coast. 



PI. 2, Figs. 14 and 20. 



Fig. 20 is probably var. perpusilla Grun. (Diat. Fr. Jos. L., PI. 4 (D), Fig. 7). 



LINEATI 

 COSCINODISCUS LINEATUS EHR. 



Valve circular, markings hexagonal, cells in parallel rows. Border narrow, cellular. 

 Blue clay and Atlantic coast. Not common. 

 PI. 3, Fig. 8. 



FASCICULATI 

 COSCINODISCUS NITIDUS GREG. 



Valve flat, markings rounded, distant, radiate, decreasing toward the border which 

 is coarsely striate. Quite variable in size and in the distance between the markings. 

 Blue clay and Atlantic coast. Common. 

 PI. 2, Fig. 18. 



COSCINODISCUS NITIDULUS GRUN. 



Valve usually not quite circular; markings smaller than in nitidus and fasciculate near 

 the border. 

 Blue clay. 

 PI. 2, Fig. 19. 

 Various intermediate forms between nitidus and nitidulus occur. 



COSCINODISCUS SUBTILIS EHR. 



Markings polygonal, irregular at the centre, but forming numerous fasciculi radiating 



