In a type dependency chain, what is the highest type in the chain called?

Hi Folks,

In the below type dependency chain BostonAreaSurfaceElevation restricts EarthSurfaceElevation which restricts Elevation which restricts xsd:integer. 

So, xsd:integer is at the top of the chain. What is that called? The start type? The head type? The beginning type? The primal type? If there is no official name, what name do you like?

/Roger

    <xsd:simpleType name="Elevation">
        <xsd:restriction base="xsd:integer">
            <xsd:minInclusive value="-999999"/>
            <xsd:maxInclusive value="999999"/>
        </xsd:restriction>
    </xsd:simpleType>

    <xsd:simpleType name="EarthSurfaceElevation">
        <xsd:restriction base="elev:Elevation">
            <xsd:minInclusive value="-1290"/>
            <xsd:maxInclusive value="29035"/>
        </xsd:restriction>
    </xsd:simpleType>
    
    <xsd:simpleType name="BostonAreaSurfaceElevation">
        <xsd:restriction base="elev:EarthSurfaceElevation">
            <xsd:minInclusive value="0"/>
            <xsd:maxInclusive value="120"/>
        </xsd:restriction>
    </xsd:simpleType>