MEXICAN FOSSIL ECHINI — JACKSON 235 



ambulacrum III is narrow, straight, inconspicuous, but widening as 

 it approaches the peristome. The posterior pair of ambulacra I 

 and V extend backward in almost a straight line and are widely 

 divergent from the anterior pair. The pores of the paired ambulacra 

 are dorsally very much alike, the inner pores being rounder and the 

 outer pores elongate. The interambulacra are very wide and are 

 highly rounded in the medium line. Dorsally there are numerous 

 perforate primary tubercles, widely spaced with secondary tubercles 

 and miliaries between them. Ventrally the primary tubercles are 

 larger, more crowded, and with numerous smaller tubercles between 

 them. In the best-preserved specimen the peripetalous fasciole is 

 in part \'isible. The apical disk is small, with five small ocular and 

 four small genital plates situated at the highest point of the test, 

 which is eccentric anteriorly. The periproct is large, situated pos- 

 teriorly and just not visible in dorsal view, clearly visible on the 

 posterior slope when viewed ventrally. The peristome is large, 

 situated far anteriorly. The plastron is prominent, elongate, ele- 

 vated, and relatively narrow. 



The holotype (U.S.N.M. no. 496281), wliich is the smallest of the 

 three knov^'n specimens, is selected as such because it is the most per- 

 fectly preserved. It measures 127 mm in length, 116 mm in width, 

 and 58 mm in height; the distance from the center of the apical disk 

 to the anterior margin is 27 mm (pi. 14 is slightly foreshortened). 

 The largest specimen measures 152 mm in length, 139 mm in width, 

 and 71 mm in height. This magnificient species is represented by 

 three specimens in remarkably perfect condition of preservation. 

 The tests are not at all compressed or distorted and show surface 

 characters exceptionally well, especially the smallest one of the 

 three. 



E. mexicanus approaches nearest to E. vaughani Jackson, ^° from the 

 Oligocene of Antigua, West Indies. It differs from vaughani in being 

 higher and more steeply rounded anteriorly, in outline more truncate 

 anteriorly, and more elongate-ovate posteriorly. In mexicanus the 

 peristome is situated farther anteriorly, and the plastron is narrower 

 and less elevated posteriorly than in vaughani. 



Lambert (op. cit.) records a specimen of E. vaughani under the name 

 Antillaster vaughani (Jackson) from near Topilla. His specimen, as 

 stated and figured, is incompletely preserved, and it is quite possible 

 that it may be referable to the new species E. mexicanus. 



Meson formation, former Corona camp site, at southern end of 

 Cerro La Puerta, Hacienda Santa Fe, Topila, Canton Ozuluama, 

 Veracruz; Corona collection, three specimens, nos. 1-3. 



» Carnegie Inst. Washington Pulil. 306. p. 96, pi. 17, fig. 2; pi. 18, figs. 1, 2, 1922. 



