Consider the following anatomical parts of crustacean: carapace, abdomen, and areola.
In much of published literature, you'll see the phrases "carapace length", "abdomen length," and "areola length" used much more frequently than what I'd consider being the more correct phrases that use adjective forms of the parts: "carapacial length," "abdominal length," and "areolar length."
A Google Scholar search for these phrases produces the following percentages of use compared to ones not using adjective forms:
"carapacial length": 3%, "abdominal length": 34%, and "areolar length": 9%.
So what's correct? Are both forms ok to use? Is this perhaps a case of widespread use of improper grammar?