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Show 48 CH.USTACEA. INAoHus, Fab., The tail is always composed of six segments; all the tarsi are nearly straight, or but slightly arcuated; the ocular pedicles are smooth, susceptible of being concealed within their fossul::e, and there is a tooth or spine, at least in the males, at the posterior ex. tremity of the latter cavities. Doctor Leach has considerably reduced the original extent of this grou p(l ). AoH..t:us, Leach. Six segments in the tail, but the four posterior tarsi are arcuated or falciform; the ocular pedicles are always salient and present a tubercle anteriorly(2). Next come those in which the epistoma is longer than it is broad, shaped like an elongated triangle truncated at the apex, and in which the origin of the mediate antennre is separated by a considerable space from the superior margin of the buccal cavity. The ocular pedicles are -always salient when the head is triangular and terminated in a point more or less bifid or entir~. STENORHYNoHus, Lam.-Macropodia, Leach. Six caudal segments in both sexes; anterior extremity of the shell bifid(3). LEPTOPODIA, Leach. Five segments in the tail of the male; one more in that of the female. The shell is prolonged anteriorly into a long, entire, and dentated point( 4). The latter Trigona differ from the preceding in the dissimilitude of their posterior feet. (1) Cancer dodecos .'J L.; lnachus scorpio, Fa b. ;-lnaclms Dorsettensis Leach 1\fa· lac.: Brit., xxii, A;-Inachus pltalangium, Fab.; Inachus doryncltus: Lenc~, lb., xxu, 7, ~;-lnaclmsleptorinchus, ejusd., lb., xxii, B; Cancer tribulus, L.? Near th.e lnncht com.es a new genus lately established by M. Guerin, called Eurypode, mmutely descr1bed and carefully figured, Mem. du Mus. d'Hist. Nat. XVI. It ap· proach:s that of Innchus, but the ocular pedicles are always salient; the post·ab· domen. JS cm~~osed of seven completely separate segments in both sexes, and the penultimate JOmt of the feet, or the metarsus, is inferiorly dilated and compressed. (2) .llchreus Cranchii, Leach, Malac. Brit. xxi, C. (3) Macropodia tenuirostris, Leach, Malac. Brit., xxiii, 1-5· Inachus longirostril? Fab. ;-Macrop. phalangium, Leach, lb., xxiii, 6. ' ( 4) Inachus sagittarius, Fab.; Leach, Zool. Misc., lxvii. DECAPOD A. 49 PAcToi.us, Leach. The four or six anterior feet simple, or without forceps. _The in- 1 t em.1 ty of the penultimate joint of the four posterwr ones t.e rna 1e x re d into a tooth forming with the last )· O·t nt a f orc~ps or 15 pro ong ' . L d' didactyle hand. The form of the shell is that o£ the eptopo ue, d t h tail presents the same number of segments; but the feet are an e · · 1 · d. · 1 1 much shorter; those of the third pair were wantmg m t 1e m IVH ua which served as the type o\ this section( 1 ). LITHODES, Lat. The Lithodes, as to the form of the first eight pairs of fee~, resemble the other Trigona; their length, however, seems progressively to increase from the second to the fourth, but the two last are very small, bent, but slightly visible, beardless, and apparently useless. The tail is membranous with three crustaceous an.d transverse n the sides and another on the end, representmg the seg- ~~eso . . mentary divisions. The eyes are app~oximated mfenorly.' T~e ex-ternal foot-jaws are elongated and sahent, and the shell Is trian_gular, extremely spinous and terminated anteriorly by a dentated pom_t. These Crustacea are peculiar to the Arctic Seas(2). Our sixth section, that of the Cnn>TOPODA(3) consists ofBrachyura remarkable for a vaulted projection of the posterior extremities of th e·i r shell under which their feet, the two anterior or the claws ex. - cepted, can be completely retracted and con.cealed. The shell IS nearly semi-circular or triangular. The superior edge of the forceps · more or less elevated and notched in the manner of a crest. In IS · f those species where they are largest, they cover the anterior part o their body, and hence the name of Coq de mer (Sea Cock), and Crabe honteux (Bashful Crab), which have been given .to some of them. One sub-genus of this section, that of JEthra bemg closely allied by other characters with the Parthenopes ?f Fabricius, the first sub-genus of the preceding section, it follows, m a natural order (1) Pactolus Boscii, Leach, Zool. Misc., lxviii. . (2) Cancer maja. L.; Parthenope maja, Fab.; Inacltus maja, Id.; Lithodes arc· fica, Leach, Malac. Brit., xxiv. See also the Maja camptschensis, Tiles., Mem. Acad. St. Petersb., 1812, V, VI. (3) Several of the Arcuata, such as the Hepati~ Mursia:, Matutre, among the swimmers, have a crested forceps, and seem to be naturally allied to the Cryptopoda, so that this section should be placed higher in the scale. The same observation applies to the last one, or that of the Notopoda, for some of them approach. the Arcuata, and others the Orbiculata and the Trigona. VoL. III.-G |
