Complex sentence: The upper surface of the disc and tail are roughened by tiny dermal denticles, which become larger towards the midline of the back and tail; one or two irregular rows of thorns are present along the midline from the head to the sting.
Split into simpler sentences: The upper surface of the body and tail are roughened by tiny dermal denticles, which become larger towards the midline of the back and tail. In addition, one or two irregular rows of thorns are present along the dorsal midline from the head to the sting.