For 10 points, name this metal that is the namesake of the series that fills the 4f orbital and which lies above actinium.
It was discovered by Carl Gustav Mosander in its oxide form, which is used today in camera lenses.  It is found in monazite and bastnasite and its isotope 138 is sometimes used with barium to date rocks, although its 139 isotope is more common.  It is often found with cerium and its name means to lie hidden, although it is one of the most common rare earth metals.  For 10 points, name this metal that is the namesake of the series that fills the 4f orbital and which lies above actinium.
It was discovered by Carl Gustav Mosander in its oxide form, which is used today in camera lenses.  
lanthanum