The cheek-teeth strongly curved, forming from the base to the summit about a quarter of a circle, the concavity being directed outwards in the upper and inwards in the lower teeth.
The forms of roof are various, but mostly they commence in a steep slope at the top, gradually flattening towards the eaves so as to produce a slightly concave appearance, this concavity being rendered more emphatic by the tilt which is given to the eaves at the four corners.
They begin on the Danube near Pressburg, surround Hungary and Transylvania in a large semicircle, the concavity of which is towards the south-west, and end on the Danube near Orsova.
By the contraction of the subumbral circular muscles the concavity of the subumbrella is increased, and as water is thereby forced out of the subumbral cavity the animal is jerked upwards.
The adductor muscles placed in the concavity of the shells act upon the long arms of the lever at a mechanical advantage; their contraction keeps the shells shut, and stretches the ligament or spring h.