The era is divided into three periods: Triassic, Jurassic, and Cretaceous.

这个时代分为三个时期：三叠纪、侏罗纪和白垩纪。

To conclude, English mainly underwent three periods of development.

总之，英语主要经历了三个发展时期。

The vocabulary, your studies are organized into periods either semesters if you have two periods per year, or trimesters or terms if you have three periods per year.

看看词汇，你的学习被分成几个阶段，如果你每年有两个阶段，就分为上下学期，如果你每年有三个阶段，就分为三学期制或学期。

The term is divided likewise into three periods, of seven years each; and in each of those periods, the rate of the Irish bounty is the same with that of the American.

任期同样分为三个时期， 每个时期七年； 在每个时期， 爱尔兰人的赏金率都与美国人相同。