▁the ▁ 1 7 th ▁century ▁Dan ish ▁astronom er ▁Nicolas ▁St eno . ▁ ▁This ▁is ▁a ▁worn ▁cr ater ▁formation ▁that ▁has ▁a ▁circular ▁outer ▁rim ▁that ▁remains ▁relatively ▁well - defined . ▁The ▁rim ▁edge ▁is ▁marked ▁only ▁by ▁a ▁few ▁tiny ▁cr ater lets . ▁The ▁interior ▁floor ▁and ▁inner ▁walls ▁are ▁nearly ▁feature less , ▁except ▁for ▁a ▁few ▁faint ▁gro ove ▁marks ▁and ▁some ▁tiny ▁cr ater lets . ▁It ▁is ▁otherwise ▁an ▁und ist ingu ished ▁formation . ▁ ▁Sat ellite ▁cr aters ▁By ▁convention ▁these ▁features ▁are ▁identified ▁on ▁lun ar ▁maps ▁by ▁placing ▁the ▁letter ▁on ▁the ▁side ▁of ▁the ▁cr ater ▁mid point ▁that ▁is ▁closest ▁to ▁St eno . ▁ ▁References ▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁ ▁▁▁▁▁▁▁▁ ▁Category : Imp act ▁cr aters ▁on ▁the ▁Moon <0x0A> </s> ▁A si ad ap inae ▁is ▁a ▁sub family ▁within ▁the ▁ext inct ▁prim ate ▁family ▁N oth arct idae ▁found ▁in ▁Asia ▁during ▁the ▁early ▁E oc ene . ▁They ▁were ▁very ▁small ▁and ▁were ▁some ▁of ▁the ▁earliest ▁ad ap iform s , ▁similar ▁to ▁cer cam oni ines ▁but ▁also ▁sharing ▁features ▁with ▁s ival ad ap ids . ▁ ▁References ▁ ▁Liter ature ▁c ited ▁▁▁▁ ▁Category : Pre histor ic ▁stre ps ir rh ines ▁Category : E oc ene ▁prim ates ▁Category : E oc ene ▁m amm als ▁of ▁Asia ▁Category : F oss il ▁tax a ▁described ▁in ▁ 2 0 0 9 ▁Category : E oc ene ▁first ▁appearances