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Sand dollars diverged from the other irregular echinoids, namely the cassiduloids, during the early Jurassic, [5] with the first true sand dollar genus, Togocyamus, arising during the Paleocene. Soon after Togocyamus, more modern-looking groups emerged during the Eocene. [1] Sand dollars are small in size, averaging from 80 to 100 mm (3 to 4 ...
Dendraster excentricus, also known as the eccentric sand dollar, sea-cake, biscuit-urchin, western sand dollar, or Pacific sand dollar, is a species of sand dollar in the family Dendrasteridae. It is a flattened, burrowing sea urchin found in the north-eastern Pacific Ocean from Alaska to Baja California .
The tests (shells) of these sand dollars are round, flat and disc-like, typically measuring 3 inches (7.6 cm) in diameter. The growth rate for this animal is between 3.5 to 6 mm/yr in the latter 5 years of their lifespan, and the lifespan is typically around 8 years. [2]
Clypeasteridae is a family of sea urchins in the order Clypeasteroida.This family was first scientifically described in 1835 by the Swiss-American biologist Louis Agassiz.. The clypeasteridae also known as the sand dollar, are round and semi-flat organisms with spines lining the underside of the body and elongated genital papillae aiding its survival and reproduction.
Dendraster is a genus of sand dollars of the family Dendrasteridae [2] within the order Clypeasteroida. The extant species in this genus are found in the northeast Pacific Ocean from Alaska to Baja California. [3] The best-known, most common and widespread species is D. excentricus. [3]
Heliophora orbicularis, also known as the West African Sand Dollar, is a small sand dollar in to the family Rotulidae, and the only species in the genus Heliophora. It, and other members of Rotulidae have been found in West African marine strata from the Late Miocene onward. Like the related Rotula , it is still extant.
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Keyhole sand dollar refers to five living species of sand dollars in the genus Mellita, plus the extinct †Mellita aclinensis.They are found on the Atlantic coasts of the Americas, ranging across the Caribbean Islands (e.g. Bermuda, Jamaica and Puerto Rico), from the southern United States at the north, to the southeastern coast of Brazil at the south.