Order 1. DECAPODA. 415 



and ciliated. The tail is composed of seven se^^iiients, but three of them are conlluent in the males. The type 

 is Cancer personatiis, Herbst., found upon the coast of England. [This genus is of very difficult location, and has 

 little real relation with Leucosia : it is more nearly allied to some of the arcuated species.] 



Leucosia,l'ab., has the carapa,x of variable form, but generally globular or ovoid, and as hard as stone; the 

 lateral antennae and eyes are very small ; the tail, large and suborbicular in the females, is generally composed of 

 four or tive, but never of seven segments. Dr. Leach cut up this genus into many others. A very few species 

 belonging to his genus Ebalia are found on the English coast. The majority of the family inhabit tropical seas. 



The fifth section, Trigona, is of very great extent, and consists of species having the carapax 

 generally irregular or subovoid, and narrowed in front into a kind of beak ; ordinarily very rough and 

 uneven, with tlie eyes lateral. Tlie epistoma, or space between the antennae and oral cavity, is always 

 nearly square, and as long as broad. The claws, at least of the males, are always large and long. 

 The following legs are very long in the majority, and occasionally the posterior pair have a form dif- 

 ferent from the preceding. The apparent number of joints in the tail varies, being seven in both sexes 

 of the majority of species ; but in others, at least in the males, it is less. Many of these craljs are 

 commonly called sea spiders. Although the number of species of this section are very numerous, onlj 

 two had been discovered in a fossil state ; one of wMch, Maia Squinado, exists at the present time in 

 the same localities. 



Latreille divides this section into sub-sections, from the number of joints in the tail, and the form of the joints of 

 the foot-jaws. Amongst those with the tail, either in both sexes, or in the females, composed of seven segments, 

 Parthenope, Fabr., is distinguished by the immense size of the claws, and the smallness of the other legs ; the 

 fingers are suiidenly bent downwards, the ocular peduncles very short, and the carapax exceedingly rough. 

 A species found on the coasts of England and France (Cancer asper, Pennant) forms the genus Eurynome, Leach: 

 the tail is seven-jointed. The other species of Parthenope are found in the Indian ocean. 



Maia, Leach, has the fingers not deflexed ; the anterior pair of legs scarcely thicker than the others, which are 

 moderately long ; the carapax has two frontal spines, and its back and sides are also armed with many tubercles 

 and spines. The typical species. Cancer Squinado, Herbst., is very common on the coasts of France and the 

 Mediterranean. It is one of the largest of our crabs, and was known to the ancient Greeks under the name of 

 Maia, being sometimes figured on their medals. [By the fishermen it is called the Thorn-back, or King Crab.] 

 Another common British species is the Cancer araneus, Lin., belonging to Leach's genus Hyas, having the 

 carapax elongate, subtriangular, subtubercled, with the lateral margins dilated into a lanceolate projection, ex- 

 ternal antennae with the first joint dilated. 



Amongst the species, which have not more than sLx abdominal segments, and the legs generally long and 

 filiform, and the third joint of the outer foot-jaw narrower than <n the preceding subsection, Hymenosoma, 

 Leach, has the carapax triangular or orbicular, depressed [and softj, and the basal joint of the lateral antennae 

 does not reach beyond the ocular peduncles. The species are small, and found in the Indian and Australian seas. 

 The British genera, Jnachus and Ach<eus, have the carapax subconvex and triangular, and their abdomen six- 

 jointed. Their four pair of posterior legs are very long, especially the pair succeeding the claws. In the latter 

 respect the British genus Utenorhynchus, Latr. {Macropodia, Leach), closely resembles them, having also the tail 

 _ SLx-jointed in both sexes, and the front of the carapax notched. The type is 



^^ ^ the very common Cancer Phalangium, Pennant. The genus Pactolus, Leach, 



characterized by having the four hind-legs furnished with a didactyle claw [has 

 been found by M. Milne Edwards to have been constructed upon a fictitious speci- 

 men in the British Museum]. 



Lithodes, Latr., is at once distinguished by having the hind pair of legs so small 



as to appear almost abortive. The type is a large crab of rare occurrence in British 



seas, named Cancer Maia, Linn. The tail is membranous ; the outer foot-jaws are 



elongated and apart ; the carapax is triangular, very spinous, and terminated in a 



V toothed spine. [This is a very anomalous genus, whose relations are difficult to 



Fi({. 3.— Slenorynchas Plialangium. (igcitle.] 



[Professor Bell and De Haan have described many new and curious genera belonging to the section Trigona : 

 the former in the second volume of the Transactions of the Zoological Society ; and the latter, in his work upon 

 the Crustacea of Japan.] 



The sixth section, Cryptopoda, is composed of a few species remarkable for having the legs, 

 except the anterior pair, concealed, when folded up, beneath the dilated lateral margin of the carapax, 

 which is nearly either semicircular or triangular; the upper edge of the claws is compressed, and 

 formed like a cock's comb. The species are exotic, and compose the two genera Calappa, Fabr,, and 

 Mthra, Leach. In the shape of their claws they resemble some of the Arcuata and Pinnipedes, such as 

 Jfepatus, Mursia, &c. ; so that this section should be placed higher in the series. The same may also 

 be said with respect to the species of the following section, some of wliich approach the Arcuata, and 

 others the Orhiculata and Trigona. 



