Serialise standard xml envelope containing variable contents

I want to create a serialisable class to represent an xml envelope that can contain arbitrary message content. Example xml (simplified) below:

<?xml version="1.0" encoding="utf-8"?>
<Envelope>
    <MessageA>
        <Url></Url>
    </MessageA>
</Envelope>

<?xml version="1.0" encoding="utf-8"?>
<Envelope>
    <MessageB>
        <Value></Value>
    </MessageB>
</Envelope>

My idea is to use a generic envelope class to handle this:

[Serializable]
[XmlRoot(ElementName="Envelope")]
public class Envelope<TContent> where TContent : new()
{
    public Envelope()
    {
        Content = new TContent();
    }
    public TContent Content { get; set; }
}

[Serializable]
public class MessageA
{
    public string Url { get; set; }
}

[Serializable]
public class MessageB
{
    public string Value { get; set; }
}

These could be serialised so:

var envelope = new Envelope<MessageA>();
envelope.Content.Url = "http://www.contoso.com";
string xml = envelope.ToXml();

However, instead of the xml message that I want (per examples above), I get the following. How can I change the classes or the serialisation process to rename the Content element to the name of the Message itself?

<?xml version="1.0" encoding="utf-8"?>
<Envelope>
    <Content>
        <Url>http://ift.tt/1yyLoY7;
    </Content>
</Envelope>