Capture and email a portion of an iPad screen display?

My initial response to your query was to say “no, it’s not” because the screen capture capability on the Apple iPad (actually, the entire iOS product line, including iPhones too) is basically full screen or nothing. On a Mac OS X system like an iMac, you can use Cmd-Shift-3 to take a full screen pic and Cmd-Shift-4 to be able to specify a rectangular subset of the screen as your captured image. But there’s no analog on iOS devices.
Until I thought about it a bit more and realized that you can most assuredly accomplish what you seek, there’s just a short intermediate step involved. As with many questions I see, I’m going to assume that you seek a solution to the question of “how do I email or share on Evernote a subset of what’s on my iPad screen?” rather than being obsessed with finding a screen capture utility that has that capability, which is not an option.
So what’s the secret? Cropping.
Let’s just have a look, shall we?
First off, here’s a pretty typical view of an iPad’s screen, with icons, etc:

To take an iPad screen capture, I simultaneously press the power button on the top edge and the home button on the face of the iPad. The screen flashes white for just a moment, then there’s a new photo in the photo album that is exactly what was on screen.
With the screen captured, go into the “Photos” app and find it. It’ll look basically the same, except there’s a toolbar along the top and a ribbon along the bottom that lets you zip between photos at lightning speed:

It’s the toolbar on top that we’re interested in. For this task, tap on “Edit” and you’ll again see the image, but this time the toolbar’s different on both the top and bottom:

On the bottom bar there’s a button labeled Crop. That’s what you want. Tap on it…

This is a selectable rectangle so tap and drag the corners to define the region you are interested in, like this:

Looks good? Tap on “Crop” on the top right, then tap on “Save” in the same spot and you’ve replaced the full screen capture with the cropped subset:

Now sending it to someone via email is a breeze. While still within the Photos app, tap on the share button (it’s the box with the arrow coming out the top):

Facebook? Yup, got that. Twitter? Same. But you want to email it, so tap on “Mail” and you’re ready to proceed:

That should get you going. It’s a bit clunky but with some practice I bet it’ll only add 15-20 seconds of extra time and just a few taps for you to be able to save or share cropped regions from your iPad screen grabs!