200 MESOZOIC AND CENOZOIC ECHINODEEMATA OF THE UNITED STATES. 



somewhat flattened ambulacral ridges alternating with broad deep interambulacral depressions, 

 the central elevation being flattened on top around the slightly depressed apical system. The 

 ambulacral ridges almost coalesce around the apical system. The apex is central or subcentral; 

 the lower surface flat or but very slightly concave from margin to center. 



The ambulacral areas are wide, slightly wider at the margin than the interambulacral areas, 

 narrowing rapidly to a point about midway between the margin and the center of the test, 

 where narrower than the interambulacral areas; dorsal portions petaloid. The petals are of 

 moderate width, elongate subelliptical, long, nearly reaching the margin, the inner half of 

 each petal elevated, tumid but somewhat flattened on top, the outer half depressed, gradually 

 broadening and merging with the flattened submarginal area; the poriferous zones are narrow, 

 the inner rows of pores nearly straight and parallel for most of their length, wide apart at the 

 ends, composed of large, round openings; outer rows diverging for about half the distance to 

 the margin then drawing near inner rows, pores slitlike; pairs of pores conjugated. The sur- 

 face of the odd petal is slightly higher than that of the anterior pair, which is slightly higher 

 than that of the posterior pair. 



The interambulacral areas at the margin are somewhat narrower than the ambulacral 

 areas, near the apex they are much narrower, almost obliterated ; they become depressed rather 

 rapidly from near the apex to about halfway to the margin, whence they merge with the flat- 

 tened submarginal area, though keeping slightly more depressed than the adjacent ambulacral 

 areas. The surface of the test is covered with small but rather conspicuous tubercles, most 

 conspicuous in the interporiferous areas. 



The apical system is central, slightly depressed below the top of the ambulacral ridges. 

 The details could not be made out on the specimen studied. 



The peristome is small, circular, central. The main ambulacral grooves are simple, straight, 

 and deep from peristome to margin; they continue as faint lines over the margin, and along 

 the middle of the ambulacral ridges nearly to the apex; two faintly defined processes are 

 given off about halfway from peristome to margin which extend to and over the margin and 

 along the upper surface between the top of the ridges and the bottom of the depressions, near 

 the suture line between the ambulacral and interambulacral plates, disappearing near the 

 inner ends of the interambulacral depressions. 



The periproct is very small, inframarginal, almost marginal. 



Related forms. — A. arnoldi is most closely related to A. whitneyi. In fact specimens of 

 each species answer to Kemond's description, and in view of the loss of Kemond's type it is 

 not possible to be sure which form was really his. Merriam, evidently influenced by Gabb's 

 figure, and before Arnold's and Pack's specimens were discovered, redescribed and figured as 

 A. whitneyi the broadly subcorneal form. Arnold's specimen, with the arch of the upper sur- 

 face commencing some distance in from the edge, was identified by him as an Astrodapsis 

 whitneyi. Pack describes and figures a specimen which is evidently identical with that of 

 Arnold just referred to but errs in treating it as a variety of A. antiselli, from which it differs 

 considerably, in fact, much more than it does from the A. whitneyi of Gabb and Merriam. 

 The present form, represented by the specimens of Arnold and of Pack, appears to be entitled 

 to recognition as a distinct species, and as the writer agrees with Merriam in regarding the 

 broadly subcorneal form figured by Gabb as most probably Kemond's A. whitneyi the present 

 species is named after Arnold, who collected the type in the Coalinga district, California. 

 A. arnoldi is readily distinguished from A. whitneyi in having the arch of the upper surface 

 commencing almost halfway in from the edge, the margin and wide submarginal area being 

 notably thin, also in having the outer portions of the petals greatly depressed and merging 

 with the submarginal area, and the ambulacral ridges somewhat broader and more flattened 

 near the apex. A. antiselli has a much thicker margin and is subdiscoidal in form besides 

 differing in other ways. A. jacalitosensis has its apical region much more depressed, and into 

 this depression extend the interambulacral depressions; the odd petal is higher and more con- 

 spicuous and the inner lines of pores of the petals diverge instead of being nearly parallel. 



