Final answer:
The untrue statement is that the swim bladder is absent in fishes, as many bony fishes have a swim bladder. Fish in Chondrichthyes have cartilaginous skeletons, while Osteichthyes typically have bony skeletons.
Step-by-step explanation:
The statement that is not true regarding the characteristics of fish is: c) Swim bladder is absent in fishes. This statement is inaccurate because many fishes, specifically those that are categorized within Osteichthyes, possess a swim bladder that helps them to control their buoyancy. In contrast, fishes in Chondrichthyes, such as sharks and rays, have a cartilaginous skeleton and do not maintain a swim bladder.
Given the provided information about the unknown fish species, since it has a predominantly cartilaginous skeleton and a swim bladder but not fused pectoral fins, the fish would not belong to Osteichthyes, which would typically also have a bony skeleton. Instead, it appears to exhibit a mix of traits from both classes but does not definitively fit the usual characteristics of either, suggesting an atypical case or a need for further examination.