sentence: The pelvic fin weakly concave (curving inwards) rear margins, and the pectoral fin's outer margin is slightly convex (curving outwards), and both fins are weakly falcate.
modified: fix grammar:  pelvic fin weakly concave (curving inwards) rear margins, and pectoral fin's outer margin is slightly convex (curving outwards), and fins are weakly falcate.