That equals about, on average, 300 to 400 lines per inch with small grooves per side.

这平均相当于每英寸有 300 到 400 条线，每边都有小凹槽。

Now like the other armor we see in these animals, it's covered with numerous small grooves that housed blood vessels.

现在跟我们在这些动物身上看到的其他护甲一样，它上面有很多小沟槽覆盖血管。

However each side of these shellac discs could only play about 3 or 4 minutes of music, and record companies realized that if they wanted a longer playing record, they would need a tougher material, one that could handle smaller grooves.

然而，这些虫胶光盘的每一面只能播放大约3或4分钟的音乐，唱片公司意识到，如果他们想要播放更长的时间，他们需要一种更坚韧的材料，可以承受较小的凹槽。