Septarian concretions are a special type of concretion. Crisscrossing ridges on the outside of septarian concretions sometimes give them the appearance of turtle shells. These ridges form when a concretion shrinks and cracks and minerals, such as calcite, are deposited in the cracks. Subsequent weathering causes the softer parts between the calcite-filled cracks to erode.
t_y_l, via Wikimedia Commons