annthoceros, put right
heard as annthoceros
spelled anthoceros
Anthoceros. One n.
What the ear did
The first syllable is short and closed, and English writes that shape with a double consonant in words like cannon and annual. The Greek anthos does not, and neither does the hornwort.
A-N-T-H-O-C-E-R-O-S
The full entry for anthoceros covers the name itself.