English: Fig. 10 & 11. Patella. "The anterior part, a is known by the interruption of the muscular impression which surrounds the central disc (d.) This interruption of the muscular impression is in the place where the head of the animal lies in the shell. The impression itself is caused by the fibrous muscle which attaches the animal to the shell. The apex in Patella, generally leans towards the anterior (a) part of the shell, and away from the posterior (p); and this circumstance has caused some mistakes, because in Emarginula the apex leans towards the posterior; and students, instead of examining the muscular impression, which is the only criterion, have only noticed the direction in which the apex turned, and concluded that to be the anterior, towards which it inclined. The lines or ribs running from the base to the apex of the shell, in the direction r, are called radiating lines; and those which encircle the cone in the direction c c, from front to back, are very properly described as concentric. The length is measured from front to back in the line e; the breadth, from side to side, in the line b; and the depth from the apex to the base."