Interested in Language
embellish a molding by carving or otherwise forming a sculpted, ornamental pattern, such as egg and dart
one may enrich the echinus of a Doric capital with the egg and dart motif, ornamental pattern, such as egg and dart
I can't understand (even looking at the example) if egg and dart refer to an ornamental pattern or a molding ....
Dictionaries are great for definitions of single words and of very commonly-used collocations. With phrases, such as the one you quoted, Google is the best place to try first. I have just Googled "egg and dart" and it answered your question completely. Now it's your turn.
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.