Echinoïdes is a clade of Echinoderms characterized by long mobile spines, vulgarly called sea urchins, sea chestnut or still Echinides for the fossil sea urchins. Echinoïdes is very plentiful in the chalk.
They present a spherical test trained of patches limestones, divided into 10 meridian regions, each trained by a double row of patches:
- 5 zones ambulacraires which carry tubular feet with suction cup or podia,
- 5 zones interambulacraires which carry mobile spines or radioles.
Every plate of a zone ambulacraire is drilled by pores by which podia passes. In every podion corresponds a pair of pores framed of a called oval border péripode.
Patches interambulacraires carry tubers which articulates radioles.
Echinoïdes is divided there:
- Regular sea urchins: the mouth (ppéristome on the test) ) is on the oral face and the anus (ppériprocte on the test) on the face aborale,
- Irregular sea urchins: the anus moves what pulls a flattening of the test and the appearance of a bilateral symmetry.
In position aborale, we find a ring of specialized patches, establishing the device apical.
Among the irregular Sea urchins, the kind Micraster offers a particular interest because of his evolution. It is an excellent example of the theory Darwinist of the creation of new sorts by the natural selection. By investigating, coat by coat, chalks of the coast, we shall notice the progressive modifications of certain characters of their test, contrary to the rough modifications which can affect other zoological groups.
Conversely, if we are perfectly attentive and expert of these subtle morphological variations, we can exactly identify the stratigraphical position of an unknown coat. Thus of the fact of the importance of this kind, he is dedicated A particular page in the kind Micraster For their description and another page dedicated in himEvolution of Micraster.
The kind Echinocorys
In the region of Newhaven, this kind of sea urchin is very common in the cliffs of upper Santonien and lower Campanien. A particular stratigraphy was proposed even from the same kind, Echinocorys scutata, with whom the variations were treated as subspecies. One of these subspecies Echinocorys scutataelevata Griffith and Brydone appears in two bands between the marl Brighton and the marl Friars Bay 1.
Illustration: link British Chalk Fossils