Number of plates of the posterior ambulacra (rows Ia and Vb) entering the subanal fasciole. Synapomorphy: plates 6, 7, 8, and 9 spreading into the fasciole in E.
Number of pore pairs of the frontal ambulacrum enclosed in the internal fasciole. Synapomorphy: approximately 50 pore pairs enclosed within the fasciole.
Arrangement of the ambulacral pores enclosed in the internal fasciole. Synapomorphy: pores arranged in irregular double series [ILLUSTRATION FOR FIGURE 1 OMITTED].
Architectural position of the fasciole within the frontal ambulacrum.
The resulting vector field [ILLUSTRATION FOR FIGURE 10 OMITTED] shows quite strong differences located along the outline of the test (frontal part; midline of interambulacra 1 and 4) and along the internal fasciole (lateral and posterior parts).
The peramorphosis corresponds to an architectural differentiation of the species that encompasses the belt of plates bearing the internal fasciole and the anterior ambulacrum.
Echinocardium laevigaster displays four pairs of ambulacral plates entering the subanal fasciole (character 12).
In the group known as spatangoids (a subgroup of irregular sea urchins), additional structures such as petals or fascioles interfere with the plate pattern, enlarging the range of morphologies.
Description: Shell globose-fusiform with a tapering base; b/l 0.43-0.47, a/l 0.47-0.56; whorls strongly convex with a slight indentation below suture, and a fairly deep suture, crenulated by rib terminations, base of last whorl only slightly constricted at top of rostrum; rostrum narrow, flat-sided, without a fasciole; aperture narrowly elliptical, more or less linear, constricted at siphonal canal, which is more or less parallel-sided, its termination oblique, shallowly indented in dorsal view; columella straight.
Axial ribs somewhat arcuate, slightly opisthocyrt on last whorl, reaching and crenulating suture, evanescing on upper part of rostrum, fasciole with strong nodules; ribs in t/s strongly rounded and slightly compressed on spire whorls, lower with sloping sides on last whorl, ribs slightly wider than their intervals, one behind lip high and varicoid, 12 or 13 axials per whorl.