Fat sea biscuit

This is a very large sea biscuit with a strong inflated test, growing to a maximum length of around 200 mm (8 in). In shape it is ovate to slightly pentagonal, with the margin thicker at the anterior end. The petaloid area is broad, the anterior (front) petal being longer than the two posterior, paired petals, which are all of equal length; the area between the pores is wide and raised above the rest of the aboral (upper) surface. The oral (under) surface is fairly flat, with a deep depression around the mouth, and with deeply indented food grooves. The anus is on the oral surface near the posterior margin. The spines are short and coarse. When alive, this species is a dark brown colour, while the bare test is whitish.