Before Iswar Chandra Vidyasagar, there waswere 16 letters in Bengali. LatterLater, Iswar Chandra said there's no use offor ৡ and ৠ for a long time. Even more, every word could be pronounced without these letters: অ০, অঃ. So he removed these four letters.
Pronunciation :
ৠ it is pronounced like Rii. Same as ঋ. But that is long while ঋ is short.
ৡ it's pronounced like Lii. Nowadays we use ল্লি. And ঌ is Li. Long and short letters respectively. But ঌ is pronounced different from লি. আধুলী it was looking like আধু৯ called Adhuli. ল্লি can't be pronounced without any letter before it. পল্লী becomes পৡ called Polli.
They are changed withto ল and ই-কার. Iswar Chandra didn't remove the letter ঌ (Li). But nowadays it's not used anymore also.
References :
- https://bn.wikipedia.org/wiki/%E0%A7%A1
- https://bn.wikipedia.org/wiki/%E0%A6%AC%E0%A6%BE%E0%A6%82%E0%A6%B2%E0%A6%BE_%E0%A6%AC%E0%A6%B0%E0%A7%8D%E0%A6%A3%E0%A6%AE%E0%A6%BE%E0%A6%B2%E0%A6%BE%E0%A6%B0_%E0%A6%87%E0%A6%A4%E0%A6%BF%E0%A6%B9%E0%A6%BE%E0%A6%B8
- https://www.quora.com/Why-were-the-Bangla-letters-%E0%A6%8C-and-%E0%A7%A1-and-the-diacritics-%E0%A7%A2-and-%E0%A7%A3-made-and-how-do-I-use-them-and-why-arent-they-used-any-longer?share=1
For reading :
- বর্ণপরিচয় (Barna Poricoy) a book by Iswar Chandra Vidyasagar where he described all the phenomenon of Bangla letters.