[44]

W O R K S OF d a r w i n : v o l u m e 1 4 other cretaceous species, and with the upper, nearly straight, portion of the occludent margin in the tertiary S. magnum and the recent S. vulgare - a fact which has been mentioned under the head of Scalpellum. The edge of the upper occludent margin forms a strongly prominent ridge, with its apex forming a slight projection; a second less prominent ridge runs from the umbo to the baso-lateral angle, and a third faint ridge runs from the umbo to a point in the basal margin, nearer to the rostral than to the baso-lateral angle. Internally there is a rather deep hollow for the adductor muscle; along the under surface of the upper occludent margin there is a slightly prominent ridge, bordered by two slight depressions.