Traditional comparisons of brain volume to body mass in dinosaurs have estimated brain size as the volume of the endocast. However, the brain of the modern reptile genus "Sphenodon" fills only about half of its endocranial volume. Some paleontologists used this fifty percent estimate in their estimates of dinosaur brain volume. Other workers have observed that details on the endocranial surface indicates that some fossil reptiles had brains that occupied a much larger portion of the endocranium. Larsson notes that the transition from reptiles to birds prevents using a set ratio from being a valid approach to estimating the volume of the endocranium occupied by a dinosaur's brain.