Looks like a Septarian Geode. A Geode has open space inside which hasn't filled in all the way. When it fills all the way up and there is no open space it is called a Nodule. A Septarian Nodule in this case :)
Septarian nodules formed between 50 to 70 million years ago. As a result of volcanic eruptions, dead sea life was chemically attracted to the sediment around them, forming mud balls. As the ocean receded, the balls dried and cracked.