There is. Look at Line 19A and the X mark in the box MD. Click this link for a better graphic:
http://www.whitehouse.gov/sites/default/files/rss_viewer/birth-certificate-long-form.pdf
Here’s the request for the documents sent by Perkins Coie.
I'm willing to believe the signature on the alleged COLB is his mother's signature, forged by someone who has practiced while referring to his mother's signature, or photo shopped from his mother's actual signature. The differences, several of which are noticeable, could easily be explained by natural variation, by changes in style over time, or by the fatigue of giving birth.