132 MESOZOIC AND CENOZOIC ECHINODEKMATA OF THE UNITED STATES. 



Scutella lyelli De Gregorio, 1890 (pars), Mon. faune eocenique Alabama, pp. 250-251, PI. XLIII, fig. 21. Conrad's 



description but Morton's figure. 

 Scutella lyelli Clark, 1891, Johns Hopkins Univ. Circ, vol. 10, No. 87, p. 77. 

 IScutella lyelli Boyle, 1S93, U. S. Geol. Survey Bull. 102, p. 263. Based on Morton. 

 Periarchus sp. a. Stefanini, 1911, Soc. geol. italiana Boll., vol. 30, p. 696. 

 ISismonMa (?) lyelli Stefanini, 1911, Soc. geol. italiana Boll., vol. 30, p. 697. 



Determinative characters. — Test variable in size, subcircular to subovate in marginal outline; 

 much depressed, less so centrally where it rises in a gently rounded, convex mound of variable 

 height; margin and wide submarginal area thin; under surface flat. Ambulacral petals sub- 

 elliptical to suboblong, extending about half way to the margin. Apex and apical system central 

 or subcentral, on the somewhat flattened summit of the tumid area. Peristome small, subcir- 

 cular; ambulacral furrows simple and straight for about half way to the margin, then forking 

 symmetrically. Periproct very small, subcircular, from one-third to slightly more than halfway 

 from peristome to posterior border. 



Dimensions. — Specimen A: Length 90 millimeters; width 90 millimeters; height 12 milli- 

 meters. Specimen B : Length 32 millimeters; width 31 millimeters; height 6 millimeters. Speci- 

 men C: Length 35 millimeters ; width 34 millimeters; height 5 millimeters. 



Description. — Considerable confusion exists in regard to this and allied forms in the various 

 collections and in the literature. Conrad as early as 1834 gave an excellent description of the 

 species but no figure. The same year Morton published a poor upper view only of a small form 

 which he called Scutella lyelli Conrad, but gave no description. Conrad's form was a large 

 specimen from Alabama, forms similar to which have easily been identified among the material 

 studied by the writer. Morton's specimen was a small one, whose present whereabouts is 

 unknown, and was from a locality which is uncertain. Desor, Cotteau, and others, basing their 

 action upon Morton's figure, have placed the species lyelli in the genus Sismondia. This is an 

 error, as Conrad's form has all the characters of Periarchus. Unless his figured specimen is dis- 

 covered, Morton's form must remain of doubtful character and is therefore placed among the 

 doubtful and unrecognized species. 



Periarchus lyelli is one of the most abundant of the American Cenozoic forms, at times occur- 

 ring in such numbers and with such constancy as to afford a valuable aid in determining the 

 geologic horizon in which it is found. The test is variable in size, ranging from 1 to 3^ inches in 

 diameter. In marginal outline it is circular or subcircular to subovate, some specimens slightly 

 others decidedly broader posteriorly than anteriorly. All around the area outside the ambulacral 

 petals it is greatly depressed and quite flat; but within the petaloidal area it rises in the form of a 

 gently rounded mound with more or less flattened summit, which ranges from \ to $ inch in height 

 according to the size and age of the specimen. Sometimes the mound approaches a subcorneal 

 form, but even then the sides are more or less convex. The margin of the test is thin and slightly 

 undulating, both characters being most pronounced posteriorly; usually there are broad shallow 

 notches opposite the ends of the posterior petals, though these may be inconspicuous or absent 

 in the smaller specimens. The flattened marginal area, from the ends of the petals to the edge, 

 is thin and wide, being about as wide as, or wider than the petals are long, and usually widest 

 posteriorly. The under surface is flat or nearly so with a broad faintly depressed ring about 

 midway between margin and peristome. The apex is central or slightly excentric anteriorly, at 

 the summit of the tumid area. 



The ambulacral areas are narrow in the petaloid region but wide at the margin where they 

 about equal the interambulacral areas. The dorsal portions are petaloid. The petals are of 

 medium size; subequal in length, the odd petal being slightly longer than the rest; subelliptical 

 to elongate elliptical or suboblong in form; slightly open at the ends; extending half or slightly 

 more or less than half the way to the margin. The poriferous zones are rather wide, almost as 

 wide as the interporiferous areas; inner rows of pores nearly straight and parallel and formed of 

 oval pores, outer rows of pores slitlike; pairs of pores conjugated. A few pairs of pores occur 

 beyond the ends of the petals. 



The interambulacral areas are large, relatively wide between the petals (usually wider 

 than the ambulacral areas), nearly uniform, and composed of large polygonal plates. The 



