Closely allied to the last-named, and belonging to the same family, is the well-known Thornback Crab (_Maia Squinado_), also a very common crab, of which we give a separate illustration. The tribe _Catametopa_ does not contain many British species, the principal being the Pea Crabs; the Floating Crab, which is occasionally washed on the south-west coast; and the beautiful Angular Crab. In these the front of the carapace is turned downwards--a feature that has suggested the name of the tribe. The pea crabs are all small, and they are parasites, living within the shells of bivalve molluscs. One species--the Common Pea Crab (_Pinnotheres pisum_) is frequently found in the Edible Mussel; the female, which is much larger than the male, being much more commonly found. Another species--the Pinna Pea Crab (_P. veterum_), infests the Pinna and Modiolus. [Illustration: FIG. 216.--THE THORNBACK CRAB (_Maia Squinado_)] [Illustration: FIG. 217.--THE PEA CRAB (_Pinnotheres pisum_)] On Plate VI. is a drawing of the Angular Crab (_Gonoplax angulata_) mentioned above, the striking form and delicate colouring of which can never be mistaken. We would, however, call particular attention to the broad and square front of the cephalothorax, with its two sharp spines, and to the length of the eye-stalks. Unfortunately for the amateur, this pretty crab is only to be found in deep water, off the coasts of Devon and Cornwall, so that here, again, the aid of the fisherman is valuable; but, as observed in the case of other deep-sea dwellers, may also be looked for in the stomachs of cod and other bottom fishes. The sex figured is the male, in which, when fully grown, the front legs are much longer than in the female. [Illustration: PLATE VI. CRUSTACEA 1. Gonoplax angulata 2. Xantho florida 3. Portunus puber 4. Polybius Henslowii 5. Porcellana platycheles] The remaining division of the crabs--the _Cyclometopa_ or Round-fronted Crabs, contains the larger number of species that may truly be described as common objects of the shore, for while some of them are well adapted for swimming, and live in the open water, the majority inhabit the shore, either between or just beyond the tide-marks, roaming about more or less freely when in the water, but usually hiding under stones or weeds, or burrowing into the sand, when left behind by the receding tide. [Illustration: FIG. 218.--THE COMMON SHORE CRAB (_Carcinus mænas_)] The members of this tribe may be known at sight by the form of the carapace, which is wide and rounded in front, and narrowed behind.