Shortly after this the dorsal median keel first appeared .
The inner ( ventral ) apetural lip became retarded while the outer ( dorsal ) lip grows into a gladius - like proostracum ( B.R. &S ) .
Shortly after this the dorsal median keel first appeared and the inner ( ventral ) apetural lip became retarded while the outer ( dorsal ) lip grows into a gladius - like proostracum ( B.R. &S ) .