124 MESOZOIC AND CENOZOIC ECHINODERMATA OP THE UNITED STATES. 



Family SCUTELLIDiE. 



Genus SCUTELLA Lamarck. 



Scutella Lamarck, 1816 (pars), Anim. sans vert., vol. 3, p. 7. 

 Scutella Lamarck, 1840, Anim. sans vert., 2d ed., vol. 3, pp. 275-277. 

 Scutella Agassiz, 1841, Mon. d'echinodermes, vol. 2, Des scutelles, p. 75. 

 Echinaraehnhis Gray, non Leske, 1825 (pars), Ann. Phil, for 1825, p. 428. 

 Echinarachnius Agassiz, 1872 (pars), Revision of the Echini, p. 107. 



The genus Scutella as here used includes the greatly depressed Scutellida? without lunules 

 or digitations; with central or subcentral apical system; homogeneous or slightly differentiated 

 plates; ambulacral furrows bifurcating; periproct marginal, inframarginal, or actinal, but less 

 than two-fifths of the way from the margin; and four or five genital pores. 



Some of the forms included here are by certain authors placed under Echinarachnius 

 Leske." This practice has not been followed by the writer because apparently Echinarachnius, 

 being founded in 1778 on Echinus -placenta Linne, should replace the pre-Linnean name Arach- 

 noides Klein with which placenta was first associated by L. Agassiz in 1841, and because, 

 even if this correction is made, the writer also questions the giving of separate generic rank 

 to the forms with a strictly marginal periproct. 



Genus SCUTELLA Lamarck. 



Scutella mississippiensis Twitched, n. sp. 



Plate LIX, figures la-f, 2. 



Determinative characters. — Test variable in size; subcircular to sub triangular in marginal 

 outline, broader than long, broadest posteriorly; the whole form greatly depressed, being thin 

 and nearly flat around a narrow marginal area within which it rises very gradually and gently 

 to the low, central apex; margin thin, with a small notch in the middle of the posterior edge; 

 under surface flat. Ambulacra! petals elliptical, extending about half way to the margin. 

 Apical system central. Peristome, small, circular; ambulacral furrows simple and straight for 

 two-thirds or more of the way to the margin, then apparently forking symmetrically. Periproct 

 very small, circular, almost at the ambitus though slightly inframarginal, at the edge of the 

 notch in the middle of the posterior margin. 



Dimensions.— -Specimen A: Length 50 millimeters; width 52.5 millimeters; height 6 

 millimeters. Specimen B: Length 52.5 millimeters; width 76 millimeters; height 8 millimeters. 



Description.— This new Scutella is one of the most abundant in the American Cenozoic 

 deposits. The writer has had opportunity to examine several hundred specimens. Nearly all 

 of these were from the Eocene deposits of Mississippi, hence the specific name. The test is 

 variable in size, ranging from less than h inch to over 3 inches in diameter and from less than 

 I to nearly \ inch in height. The marginal outline is also quite variable, ranging from circular 

 to subcircular in the younger forms to subtriangular and even transversely elliptical in the 

 larger forms. The whole form is much depressed, being thin and nearly flat around a narrow 

 marginal area, although slightly more so posteriorly than elsewhere, from which it rises gradu- 

 ally and gently to the low, central apex. The margin is thin and faintly undulating, with a 

 small notch in the middle of the posterior edge; the under surface is flat except around the 

 peristome, where it is slightly concave. The apex is central or subcentral, at the summit of the 

 broad central tumid area. 



The ambulacral areas are narrow in the petaloid region but wide in the marginal region 

 where they are wider than the interambulacral areas. The dorsal portions of the ambulacra 

 are petaloid. The petals are subequal in length, the odd petal slightly longer and narrower 

 than the rest; elliptical in form, slightly open at the ends and extend about half way to the 

 margin. The poriferous zones are rather wide, about equal to the interporiferous areas, though 



