26 THE DIATOMACE.E OF PHILADELPHIA AND VICINITY 



AULACODISCUS EHK. (1844) em. RATTR. (1888) 

 (aulax, a furrow, and discus) 



Valve usually circular, plane or with an elevated zone, frequently inflated beneath 

 the processes; central space irregular or rounded, sometimes absent; markings granular, 

 radial, sometimes in a reticulum. 



The genus comprises more than one hundred species most of which are fossil, and is 

 represented in this locality by a single form, A. argus, included by Rattray in his section 

 "Retiformes," distinguished by the presence of a reticulum. 



AULACODISCUS ARGUS (EHR.) SCHMIDT 



Frustule in zone view elliptical. Valve circular, 125-190 n in diam., closely covered 

 with two kinds of markings, one, a mesh of large, radiating, angular cells, the outer plate, 

 and the other, radiating rows of circular granules with hyaline spaces intervening and closer 

 near the border, forming the inner plate which can occasionally be seen detached. Central 

 space absent. The walls of the angular cells are crossed with fine lines and are probably 

 composed of granules compressed so closely as to produce partial opacity, the depth of 

 which depends in a measure not only on the superposition of the two plates, but on the 

 relative closeness and thickness of the cell-walls. In a fully-developed specimen the effect 

 is to produce more or less triangular cells containing three or four granules. In some cases 

 the opacity is so great as to render detail invisible. 



In the figure the valve is supposed to be divided into three sectors, illustrating at "a" 

 the lower plate> at "c" the combination of the upper and lower plates, and in the other 

 sector the cellular mesh of the upper plate. Processes, usually three, quite robust and in- 

 serted at from one-fourth to one-fifth the length of the radius from the border which is 

 striated on the inner side. A form with four processes is found in the lower blue clay. 



Tripodiscus argus Ehr. 



Eupodiscus argus (Ehr.) Wm. Sm. 



Not uncommon in the blue clay. 



PI. 4, Fig. 8. 



ACTINOCYCLUS EHR. (1837) 

 (actis, a ray, and cycles) 



Valve circular or elliptical; surface flat at the centre, sloping toward the border. Cen- 

 tral space usually evident, rounded or irregular. Markings rounded, granular, punctiform, 

 in radial, or nearly radial, rows, sometimes fasciculate. A nodule, more or less evident, is 

 found near the border which is usually striate. 



Chromatophores round discs or granules. 



ANALYSIS OF SPECIES 



Valve circular, rows radial, hyaline lines at the border barkleyi 



Valve circular, rows fasciculate moniliformis 



Valve elliptical ellipticus 



The nodule is generally supposed to be a thickening of the cell-wall, and, in the opin- 

 ion of Rattray, a projection outward, but "whether there may not be at the same time a 

 slight inward protuberance is difficult to determine," though, as a rule, he seems to "think 

 there is not." 



