The Echinoid Directory

 Spatangoids with:

• episternal plates biserially offset;
• labral plate longitudinally elongate; flask to wedge-shaped;
• peripetalous fasciole; no other fascioles ever present;
• Anterior ambulacrum with suckered tube-feet aborally.

Lower Cretaceous (Albian) to Recent; worldwide.

Hemiasterids have a single fasciole that passes aborally around the tips of the petals. The peripetalous fasciole found in Schizasterids superficially resembles that of hemiasterids, but has a different origin, as first pointed out by Mortensen (1950). In schizasterids the first fasciole to form is a marginal fasciole that crosses Iamb. plate 5.a.4/5.b.4 beneath the periproct. Only later in development does a posterior branch form that runs aborally between the periproct and apical disc. In Hemiasterids the peripetalous fasciole is primary and crosses the anterior interambulacra well above the level of plate 4.

Almost all taxa have the primitive ethmophract apical disc structure and the more primitive members also have asymmetrical sternal plates.