168 MESOZOIC AND CENOZOIC ECHINODEKMATA OF THE UNITED STATES. 



posteriorly, sometimes depressed subcorneal; under surface with a deep and more or less long 

 and flaring transverse depression around the peristome. Ambulacral areas wide at the ambitus, 

 dorsal portions petaloid, with a slight constriction at the ends of the petals. Apex forward 

 of the center. Apical system central or subcentral. Peristome pentagonal, opening at the 

 bottom of the depression in the under surface. Periproct very small, circular, inframarginal. 



Dimensions. — Specimen A: Length 38 millimeters; width 33 millimeters; height 17 milli- 

 meters. Specimen B: Length 38.5 millimeters; width 31 millimeters; height 17 millimeters. 



Description. — It is thought highly probable that we have here the same species which Conrad 

 described in 1S50 under the name of Discoidea lialdermani, as some of the specimens studied 

 came from near the same place that Conrad's form did and correspond fairly well with his 

 figures and description. The type can not be located; and as Conrad did not figure the under 

 surface of his form there is some doubt regarding the position of the periproct and the presence 

 of a pronounced peristomial depression. The fact that Conrad assigned his species to the genus 

 Discoidea, points to an inframarginal position for the periproct. Should Conrad's type ever be 

 found, and prove to be different from the present species, then it will be necessary to give a new 

 name to this Oligopygus. 



This species appears to be the most abundant and also the most variable of the genus. 

 The writer has had the opportunity of studying over 35 specimens, including both young 

 and adult forms, varying in size from three-eighths inch to 1J inches in length. The test is 

 unusually thick. The marginal outline varies from regularly oval to subpentagonal, the former 

 being more usual in the smaller and younger, and the latter in the larger and older forms, and is 

 slightly undulating, which is chiefly due to broad and shallow creases in the middle of the inter- 

 ambulacral areas. The margin is rounded and inflated, the anterior end usually more so than 

 the posterior, giving that end a bulging appearance in a forward direction, though at times the 

 two ends are nearly equally inflated. The upper surface is somewhat convex, but more or less 

 depressed and flat on top, sometimes more elevated anteriorly than posteriorly, sometimes 

 depressed subcorneal in form ; under surface tumid, but more or less flattened, and with a deep 

 transverse depression around the peristome. The apex is subcentral or excentric anteriorly. 



The ambulacral areas are wide at the ambitus, from which they gradually narrow to the 

 peristome and apex, with a slight constriction at the ends of the petaloid portions; the anterior 

 paired ambulacra curving posteriorly on approaching the peristomial depression; dorsal por- 

 tions varying from subpetaloid to slightly petaloid, the odd petal being slightly longer than the 

 others, which are equal or nearly equal in length. Poriferous zones rather broad, gradually 

 diverging toward their extremities, then converging slightly but still wide apart at the ends; 

 outer edges slightly depressed, forming ten more or less well defined linear grooves on the upper 

 surface of the test; the pores equal or nearly so, small, elliptical, and conjugate. There are also 

 several pairs of very small pores near the outer end of each plate from the ends of the petals 

 to beyond the ambitus. The interporiferous areas are slightly tumid. 



The apical system is central, or slightly excentric anteriorly. The madreporite is large, 

 tumid and subcircular, and extends to and between the posterior radial plates. There are 

 four genital pores, situated at the edge of the madreporite, the posterior pair being farther apart 

 than the anterior; and there are five small radial plates, each perforated by a minute pore. 



The peristome is pentagonal and opens at the bottom of a rather deep transverse depression in 

 the middle of the under surface. This depression varies in length from one-third to one-half the 

 width of the test, and in depth from one-fourth to one- third the height of the test; and is more 

 or less open and flaring in form. 



The periproct is very small, uniform, circular, and inframarginal. 



Numerous small, uniform, imperforate tubercles, surrounded by deep scrobicules, are scat- 

 tered irregularly over the entire surface of the test. 



Related forms. — Oligopygus Jialdermani resembles 0. wetlierhyi . and 0. floridanus but is 

 readily separated from both by the inframarginal position of its periproct. Of foreign forms, 

 0. ovum-serpentis (Guppy), from supposedly Eocene deposits of Trinidad, is closely allied but 

 has its periproct farther from the margin. 



