原句应是这个样子：

but he could not maintain himself in a continued conversation with a lady reared as Mary was.

我觉得，这个句子没有什么特别难懂的地方。

rear 的意思是“养育；抚养”，在这里是过去分词作定语，修饰a lady。

as Mary was 是“方式状语从句”，修饰过去分词reared。

原句其实是在as Mary was  之后省略了reared：像玛丽被抚养长大那样。

maintain himself in a continued conversation with a lady：继续和一位女士交谈下去。