Name two mammals which lay eggs. Between which two phyla do they serve as connecting link? Name a mammal which gives birth to poorly developed young ones.

(N/A) The two mammals that lay eggs are the $Echidna$ (spiny anteater) and the $Platypus$ (duck-billed platypus).
These egg-laying mammals,known as monotremes,serve as a connecting link between the class $Reptilia$ and the class $Mammalia$ because they possess characteristics of both groups (e.g.,laying eggs like reptiles but having mammary glands like mammals).
$A$ mammal that gives birth to poorly developed young ones is the $Kangaroo$ (a marsupial).