218 MESOZOIC AND CENOZOIC ECHINODERMATA OF THE UNITED STATES. 



Dimensions. — Length 51 millimeters; width 39 millimeters; height 8 millimeters. 



Related forms. — Judging from Pack's figures and description A.fernandoensis is very closely 

 allied to A. antiselli Conrad, the most notable points of resemblance being the oval outline, the 

 slight pointing of the posterior end, the rounded edges and the slight convexity of the upper 

 surface. The points of resemblance are so striking as to raise the question, whether A.fernando- 

 ensis ought not to be regarded as a synonym of A. antiselli; but as Pack does not state that the 

 margin of A. fernandoensis is very thick and as the figures suggest a greater contrast between 

 the elevation of its ambulacral ridges and the depth of its interambulacral depressions than 

 occurs in A. antiselli, decision is reserved until authentic specimens of Pack's form can be 

 examined. 



Locality. — Elsemere Canyon, Los Angeles County, Cal. 



Geologic horizon. — Fernando formation, lower Pliocene beds. 



Collection. — University of California (A, B). 



Family CLYPEASTRHWE. 



Genus DIPLOTHECANTHUS Duncan. 

 DlPLOTHECANTHTIS DALLI Twitchell, 11. Sp. 

 Plate XCIX, figures 2a-b; Plate C, figures la-b. 



Determinative characters. — Test large; pentagonal in marginal outline, longer than broad, 

 broadest opposite the ends of the anterior petals, pointed anteriorly, posterior end truncated 

 centrally; upper surface irregularly convex, high, highest back of center, whence sloping gently 

 and in a straight line to the blunt, tumid anterior edge and quite steeply and in a straight line to 

 the thinner, wedge-shaped posterior edge ; under surface deeply concave, the concavity beginning 

 near the margin and increasing at first gradually then rapidly to the center. Ambulacral petals 

 large, broad, very tumid, almost reaching .the margin, nearly closing, the posterior pair slightly 

 longer than the odd petal which is slightly longer than the anterior pair. Apical system central, 

 sloping downward anteriorly; five genital pores a short distance from the slightly depressed 

 madreporite. Peristome large, slightly excentric posteriorly, subpentagonal, deeply sunken; 

 ambulacral furrows simple, straight, reaching the margin. Periproct rather large, subcircular; 

 inframarginal, almost marginal. 



Dimensions. — Length 112 millimeters; width 92 millimeters; height 50 millimeters. 



Description. — Only a few specimens of this handsome clypeastroid form have been dis- 

 covered. The test is large; pentagonal in marginal outline, the corners of the pentagon being 

 rounded and the sides straight or faintly concave; longer than broad, broadest opposite the ends 

 of the anterior petals, pointed anteriorly, posterior end truncated centrally; margin very thick 

 and rounded opposite the petals, thinner between them, thinnest along the wedge-shaped pos- 

 terior edge. The upper surface is irregularly convex, high, relatively higher than in other 

 American clypeastroid forms. The apex is slightly excentric posteriorly, being located at the 

 posterior part of the apical system from which point the upper surface slopes gently (the angle 

 being about 30° from the horizontal), and in a straight line to the wedge-shaped posterior margin. 

 The outline along a longitudinal median section is therefore angular at the apex. The under 

 surface is strongly concave; the concavity beginning near the margin and increasing gradually 

 for about half the way, then very rapidly for the rest of the way to the deeply sunken peristome, 

 where it is about one-half the height of the test. The rapid increase in the concavity begins 

 somewhat farther from the anterior end than it does elsewhere. 



The ambulacral areas are wide, the dorsal portions petaloid. The petals are very large and 

 conspicuous, broad, very tumid, almost reaching the margin, nearly closed at the ends, the pos- 

 terior pair slightly longer than the odd petal which is slightly longer than the anterior pair. The 

 poriferous zones are wide, narrowing and converging rapidly at the ends so as to almost com- 

 pletely inclose the interporiferous area, the inner row of pores notably higher than the outer row; 

 pores round, subequal, pairs of pores conjugated. The interporiferous areas are very wide, very- 

 tumid, the inner portions uniting to form a narrow tumid ring around the madreporite. 



