The left-side individual, with a Cassius Blue from the LRGV for comparison.

I have a theory about IDing this species from Cassius Blue:

Note the arrangement of dark marks underlined in pink:

For Marine, the dark block in the fourth inter-veinal wedge (the wedge labeled with a green "4") is usually in-line with the outer three blocks; this individual Marine has a slight "uptick" in the 3rd inter-veinal block just before it meets the 4th, but the 4th inter-veinal block is still mostly in-line with the others.

For Cassius, the 4th inter-veinal block is sharply offset basally such that its upper/outer corner touches (or is separated from) the inner/lower corner of the 3rd inter-venial block.

I have been checking this feature for many years, and I feel that it is a very reliable means of distinguishing between the two taxa.