Style internal links

I would like to apply different css style, such as text-decoration, to internal links.

Here is my present link code (b_text is the post content styling class):

.b_text a, .link { text-decoration: none; color: #3F474E;}

I have tried the following, but either it doesn’t affect the links, or if I insert .link, it changes the style of all links:

.internal a { color: #2C40DA !important; text-decoration: underline !important; font-weight: normal !important;}

What might be the right combination of classes to affect only internal links?