Late Miocene freshwater cockles from the Croatian Natural History Museum (CNHM) were studied in order to recognize the size patterns within the population and reveal the possible subjective approach to the collecting process (size or preservation quality preferences). The most abundant taxa, Lymnocardium baraci Brusina, L. diprosopum Brusina, L. inflatum (Gorjanović - Kramberger), L. mayeri Hörnes, L. riegeli (Hörnes), L. rogenhoferi Brusina, Prosodacnomya vutskitsi (Brusina) and Pseudocatillus simplex (Fuchs), from the localities: Okrugljak (SW Medvednica), Glogovnica (S Kalnik) and Glogovac (Bilogora), were chosen for comparative studies. Relations between the length (L) and height (H) (=surface) and length (L)/height (H)/width (W) (=volume) values were calculated for each taxon and compared within a single species and within the cockle populations from the same locality. Calculations yielded the following results: (1) Volume (L/H/W) compared to surface (L/H) studies proved to be the better choice in population analyses; (2) size distribution values group independently of species dimensions, but can be well correlated within the finding-sites; (3) cockles from Bilogora-Kalnik area show normal value distribution, while all taxa from Okrugljak locality present a similar slight asimmetry of size distribution; (4) such distribution probably points to the different taphonomical processes in these two depositional basins. This research has proved that the CNHM Late Miocene cockle collections can be successfully studied from all paleontological aspects, even when field outcrops are no more available, because fossils were collected objectively, taking all available specimens from the site, regardless of their size or preservation quality.