On the second photo the black furrow in the center is the opening between the shells as seen from the end opposite the hinge. The top photo is a side view (the right side of the opening in the second photo). For example, rotate the third pic 90 degrees horizontally to the left and you have the same perspective and the second photo, rotate another 90 degrees and you have the same perspective and the first. As for that growing on the shell I am not certain if it is a red algae or a higher plant, I am assuming it is an aglae.