Here, in the only answer, it's a simple arrow: How to present an abstract class in UML class diagram
But here, it's similar to the interface arrow: http://www.cems.uwe.ac.uk/~jsa/UMLJavaShortCourse09/CGOutput/Unit9/unit9(0809)/page_03.htm
I'm thinking it should be the same arrow as class to interface arrow of the diagram; is that right?