Echinoid Radioles, Radiolus penna, Cidaris trigona. 219 
side (figs. 406, 409), the handle of the shaft merges gently into the vanes, and the 
back forms an equable curve. On the adapical side (figs. 404, 407, 411), the 
handle is continued down the shaft as a mid-rib, attaining a sagittal diameter of 
2-4 mm. at 8°38 mm. from the annulus, and having a transversal diameter of about 
one-third the total width of the shaft. This latter gradually increases to an estimated 
width of 5 mm_ The vanes join the mid-rib on this side by gentle concave curves, 
almost the same as the convex curve of the mid-rib itself, but their outer margins do 
not rise so high as the median line of the mid-rib. (See sections, figs. 405, 408, 
410, 412). 
In a there is more tendency to a median ridge along the adoral side than on 
b, c or d. There is no trace of ornament on any part of a, b, or d; but c, which 
is less weathered, shows a suggestion of shagreen towards the distal end of the 
adoral surface. In e the mid-rib bears a few small scattered pustules (fig. 411). 
In g there is slight ribbing at the distal end of the adoral side. 
Relations of the species. — Cidaris bicarinata Kurs. differs from 
Radiolus penna in having a hollow shaft with vanes reduced to mere keels, which 
have little more effect than to give to a créss-section of the shaft a sub triangular 
outline (fig 434). C. alata in some of its forms comes nearer to Radiolus penna, but 
the mid-rib is not so sharply distinguished and there is more pronounced ornament. 
Without asserting that these radioles were borne by a species distinct from any to 
which a name has ever been applied, we may regard them as a well-defined type 
of radiole, at present incapable of reference to any known species. 
«Cidaris» trigona. 
(Piate XIII, figs. 413—416, and Plate XVII, fig. 452) 
1841. Cidaris trigona MUnster, Beitr. z. Petrefactenk. IV, p. 44, pl. iii, f. 15 a, b. 
1848. Cidaris imbricata CORNALIA, Notiz. geo-min. sopra alcune valli... d. Tirolo, p. 40, pl. ii, 
f. 4 a—c, a’. 
1848. Cidaris truncata Corna.i, Notiz. geo-min. sopra alcune valli... d. Tirolo, p. 39, pl. iii, f. 3 a, 6. 
1855. Cidaris trigona MOnsv., J. KoECHLIN-SCHLUMBERGER, Bull. Soc. Geol. France (2), XII. p. 1063. 
1865. Cidaris trigona Minst., G. C. LauBe, Denkschr. Akad. Wiss. Wien, Math.-Nat. Cl. XXIV, 
Abth. 2, p. 285, pl. viii b, f. 6. 
1875. Cidaris trigonus Mtnsr., F. A. QUENSTEDT, Petrefactenk. Deutschlands, III, p. 196, pl. Ixviii, 
f. 83—85. 
1900. Cidaris trigona Mtwsy., E. K, Hesse, N. Jahrb. Mineral., Beil.-Bd. xiii, p. 229. 
1904. Cidaris trigona Mtnst., F. Broiti, Palaeontogr. L, p. 156, pl. xvii, f. 42—44. 
Diagnosis. — A Cidaroid in which the primary radioles have a coarse 
micro-structure of irregular prisms, running vertically in the axial complex, but 
bending outwards near the sides, and sealed on the surface by a thin cortex; 
the shaft has a short smooth handle, and massive blade thickening distalwards, with 
one flattened or concave face, and two other faces meeting in a rounded back; 
ornament usually imbricate on the flattened face but pustulate on the other faces ; 
distal end swollen and separated from the sides by a distinct edge, sometimes raised 
in a rim; base relatively small, with smooth acetabular margin and annulus, and 
very low collerette. 
Holotype. — Of the two specimens figured by Mtnsrer (1841, Taf. III, 
f. 15) and now in the Palaeontological Museum, Munich, the original of his f. 15 
is hereby selected as holotype. 
