plate, the coalesced ventral plates of the last five segments of the trunk are distinguishable, and three earlier segments are obscurely represented. It is never entirely linear. The vulvæ of the females are generally placed upon it, but in some groups are transferred to the basal joints of the ante-penultimate legs. The pleon is of subordinate size, usually reflexed against the concavity of the plastron, in the male generally narrow and pointed, with only one or two pairs of pleopods, in the female broad, with four pairs of pleopods. The basal joint of the first antennæ contains auditory hairs but no otoliths.

In this definition the Anomura apterura are included. To the dry bones of definition must be added an even less appetising explanation of terms in common use for the description of genera and species. The orbital regions of the carapace speak for themselves as being those which contain the eyes. The front' lies between them. Behind it on the under surface are the fossettes of the first antennæ, followed in the median line by the epistome, the buccal or oral frame, and the sternal plastron. The second antennæ are placed outside of and a little behind the first. The 'hind margin' of the carapace separates the trunk from the pleon, and lies between the first joints of the last pair of trunk-feet. Between it and the orbits are the lateral margins, each of which is subdivided into an antero-lateral and a postero-lateral portion forming, when not continuous, the epibranchial angle. The dorsal surface of the carapace is marked by several grooves corresponding with the insertions of muscles underneath, and also forming the boundary lines of regions which roughly coincide with the positions of important internal organs. Along the centre lie the gastric, cardiac, and intestinal regions, respectively over the stomach, heart, and intestine. The hepatic regions over the liver flank the gastric region on either side in front, and behind these lie the two branchial regions, the 'cervical groove' being that which separates the gastric and hepatic regions from the cardiac and branchial. On the under side the pterygostomian regions, 'the wings of the mouth,' lie between the antero

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lateral margins of the carapace and the buccal frame. Milne-Edwards remarks that the grooves are often emphasised about the middle of the carapace, so as to produce the appearance of the capital letter H, the transverse

FIG. 2.-Ethusa mascarone (Herbst). [Herbst]

line being the upper boundary of the cardiac region. In some cases the grooves are so arranged as to represent very strikingly a human countenance or the caricature of one, as in the Masked Crab of Great Britain and the grimacing Ethusa mascarone of the Mediterranean, which is here shown as depicted by Herbst. Such likenesses the old writers were not at all disinclined to accentuate.

The Brachyura are divided into tribes, in regard to which, however, there is not at present any absolute agreement among naturalists. We shall here arrange them under the names Cyclometopa, Catometopa, Oxyrrhyncha, Oxystomata, Anomala. It is melancholy, but scarcely avoidable, that an alternative list of names should have to be mentioned, for these tribes in the same succession may be called--Cancroidea, Ocypodiidea, Maioidea, Leucosiidea, and Anomura apterura. The subjoined table will be useful for reference.

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CHAPTER VI

TRIBE I.-CYCLOMETOPA

THE name literally means 'those of a circular forehead.' In these Crustacea the carapace is often of a breadth greater than the length, wide and regularly arched in front, more rarely quadrate or suborbicular, but not rostrate. The epistome is short, transverse. The first antennæ are in general transversely folded. The third maxillipeds have the fifth joint articulated at the apex or the inner front angle of the fourth (except in Pirimela). There are nine pairs of branchiæ, with their efferent channels opening at the sides of the endostome or palate. The verges of the male are inserted at the bases of the last legs of the trunk.

Milne-Edwards states that the different ganglia of the trunk form a sort of circular ring, of which it is often easy to distinguish the constituent elements, and that the two halves of the liver remain distinct without a median lobe.

The tribe has been subdivided into four legionsCancrinea, Cyclinea, Corystinea, Thelphusinea, in defining which I shall follow the safe guidance of Mr. E. J. Miers, as afforded in his report on the Brachyura collected by the Challenger.

Legion 1.-Cancrinea.

The buccal cavity is usually well defined. The flagella of the second antennæ are not greatly elongated. The seventh joint in the walking-legs is generally unarmed. It is either stiliform or in the last pair expanded into an

ovate swimming organ. The species are marine or

littoral.

This legion contains four families-Cancridæ, Trapeziidæ, Portunidæ, Podophthalmidæ.

Family 1.-Cancride.

The carapace is commonly transverse and convex, with the antero-lateral margins arcuate, and armed with several lobes, teeth, or spines. The 'front' is of moderate width, in general not projecting over the first antennæ and the bases of the second, the latter being seldom excluded from the inner hiatus of the orbits.

In this family are included about half a hundred genera, some widely and conspicuously distinct, others separated by fine and almost inappreciable differences. Thus Mr. Miers observes of Xantho (Leach, 1813), that 'it is connected by almost insensible gradations on the one hand with Lophoxanthus and Xanthodes, on the other with Panopeus and Eurypanopeus.' Quite recently the genus Panopeus, H. Milne-Edwards, 1834, has been reviewed by James Benedict and Mary Rathbun. They recognise in it thirty-eight species, and re-include within its boundaries Eurytium, Stimpson, 1859, and Eurypanopeus, A. Milne-Edwards, 1880, considering that they have been separated from the parent genus on grounds insufficient or untenable. It will, however, be quite beyond the range of such a manual as this to enter into all the minutiae of generic distinctions. Far less can the characters of innumerable species be discussed. Only the specially typical or the specially anomalous forms may court a passing attention. Here and there a comparison, a description, a comment, may indicate the variety of details upon which classification is founded, or may suggest the endless opportunities for the exercise of keen eyes and acute minds, which the subject provides.

Those whose scientific zeal is limited to the desire of having the specimens in a cabinet rightly arranged and ticketed with their proper names are often puzzled and