Very difficult to distinguish this from the Cassius Blue, except by its range. The Cassius is typically only found in the lower South Texas area adjacent to Mexico, where they do overlap with the Marine Blue. Refer to Butterflies though Binoculars - The West, page 106, for positive ID regarding dorsal side of the forewing, which must be extended to reveal the 4th dark band in from the outer wing margin. The Marine Blue overlaps the region and habitat of the Western Pygmy Blue, and is only slightly larger.
catbird8