I keep coming across ▼ after entries in the Sanseido dictionary, such as this one. I can’t find what it is anywhere. Is it for collocations? If I put them together, like from the example, 動物図鑑 doesn’t show in a dictionary, but does bring up relevant pictures on Google Images.
The marks aren’t standard though. In some other dictionaries you’ll see an X for non standard readings.
There are other triangles too, that aren’t black, those usually mean that the kanjis are jojo Kanji.
Usually (at least in EPWING files) there will be a page explaining the symbols and abreviations used in the dictionary