XVII] THE COMPARISON OF RELATED FORMS 1057 



the carapace conforms to a triangular diagram, more or less curvi- 

 linear, as in Fig. 513, 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 



, ^ 



Fig. 513. Carapaces of various crabs. I, Geryon; 2, Corystes; 3, Scyramathia; 

 4, Paralomis; 5, Lupa; 6, Chorinus. 



part is longitudinally extended as compared with the posterior. 

 A system of slightly curved and converging ordinates, with ortho- 

 gonal and logarithmically interspaced abscissal hues, as shewn in 

 the figure, appears to satisfy the conditions. 



In an interesting series of cases, such as the genus Chorinus, or 

 Scyramathia, and in the spider-crabs generally, we appear to have 



