Costa

The costa on a moss leaf is similar to a midrib on a "normal" leaf. In moss, some costas are two pronged, others are extremely short. Some costas extend past the leaf other end shortly after beginning. Some moss don't even have a costa! None the less, they can prove a useful identifying character.

Lamellae

Lamellae are a cool structure on some moss leaves. They are used to maximize photosynthesis. In some cases, the leaves curl completely over the lamellae and create a "greenhouse" where air circulation can be regulated! This is a picture of Polytrichum commune, with "normal" lamellae.

Muscinae

The most beautiful rendition of Moss I have seen in awhile. The 72nd plate from Ernst Haeckel's Kunstformen der Natur (1904), depicting organisms classified as Muscinae.