found easy and significant, even, in many cases, down to minute details, such as the number and situation of the marginal spines, though these are in other cases subject to independent variability.
If we choose, to begin with, such a crab as Geryon (Fig. 369, 1), and inscribe it in our equidistant rectangular co-ordinates, we shall see that we pass easily to forms more elongated in a transverse {745} direction, such as Matuta or Lupa (5), and conversely, by transverse compression, to such a form as Corystes (2). In certain other cases the carapace conforms to a triangular diagram, more or less curvilinear, as in Fig. 4, which represents the genus Paralomis. Here we can easily see that the posterior border is transversely elongated as compared with that of Geryon, while at the same time the anterior part is longitudinally extended as compared with the posterior. A system of slightly curved and converging ordinates, with orthogonal and logarithmically interspaced abscissal lines, as shown