What is a median process on the front part of the neural arch of the vertebræ of most snakes and some lizards, which fits into a fossa, called the zygantrum, on the back part of the arch in front called?

zygosphene — A median process on the front part of the neural arch of the vertebræ of most snakes and some lizards, which fits into a fossa, called the zygantrum, on the back part of the arch in front. (Webster's 1913.)