The observed specimen belongs to the species Gasteracantha arcuata. It has two long curved spines on its back and black/dark gray color all over. According to the sources on the internet[*], this species is found in Malaysia, Indonesia, Thailand, India and China. Sightings of this species in Bangladesh is probably not well documented. From my last expedition to Lawachhora forest however, it is evident that this species is abundant in that area.
I'm planning an expedition for collecting more information on this species and documenting its behavior. Hopefully I'll meet these critters in Lawachhora again at the end of December, 2010.
Here are some more photos of this species found on the internet.