Bivalves are easily found along Central Java’s coast and are an ecologically and economically important marine group of organisms. One of them was Placuna placenta, the Window Pane Scallops. It is commonly consumed, and the shell of this species is manufactured for many handicrafts. The present work reports the morphometric and size class of P. placenta in the Wulan estuary, Demak. The samples were collected using a dredge from seven sampling sites and then were measured for their shell length. The P. placenta’s shell is considerably inequivalve, thin, fragile, almost equilateral, rounded to saddle-shaped, and extremely compressed laterally. The umbones are low and submedian, and the outer surface is relatively smooth, with lamellate growth lines and occasionally fine radiating threads. The density of P. placenta was found to be varied with the sampling sites. The highest density coincided with high chlorophyll-a and the small and large class groups living in sampling sites 1, 2, and 7 for the small and 5, 6, and 10 for the big, respectively. The existence of the small size group in some sampling sites showed that it was the nursery ground for this species. Hence, it is urged to be conserved to maintain its stock.