437 



PORPITA. 



PORTUNID^E. 



438 



Felspar of a granular texture, without crystals imbedded, is Clay- 

 stone ; with imbedded crystals of felspar it is Porphyry. Hornblende 

 mixed with uncrystallised felspar makes some Greenstones ; with 

 imbedded crystals of felspar this becomes Greenstone-Porphyry. 

 Hence, by this mode of designation, we have Pitchstone-Porphyry, 

 Greenstone-Porphyry, Bal sal tic-Porphyry, Felspar-Porphyry, and even 

 BO vague a term as Trap-Porphyry ; and, on the other hand, more 

 correctly, Porphyritic Pitchstone, Porphyritic Claystone, Porphyritic 

 Greenstone, Porphyritic Basalt, and Porphyritic Granite. 



Among volcanic rocks, Porphyritic Trachytes are common. 



Fine examples of porphyry (crystals of felspar in a base of uncrys- 

 tallised felspar) occur in Scotland, at Inverary and ill Ben Xevis ; in 

 Cumberland, on Armboth Fell ; and in Cornwall, commonly under the 

 title of Eurite. 



PORPITA. [ACALEPUJI] 



PORPOISE. [CETACEA.] 



PORTLAND OOLITE, a term in Geology, synonymous with 

 Epioolite of Brongniart, and Upper Oolite of some English geologists. 

 [OOLITE.] 



PORTULA'CE J5, Pttrtlanes, a small natural order of Polypetalous 

 Exogenous Plants withhypogynous stamens; distinctly characterised by 

 having two sepals, five petals, and a central placenta, whose seeds con- 

 tain a curved embryo, lying upon mealy albumen. Their fruit is com- 

 monly 1-celled, but the edges of the carpels are sometimes so much 

 inflected as to press upon the placenta and divide the interior into cells. 

 They are annual, perennial, half-shrubby or shrubby plants, with entire 

 more or less succulent leaves, rarely furnished with stipules, and 

 with flowers which are often very showy, although more frequently 

 inconspicuous. Their prevailing colour is purple or scarlet. They 

 occur in all the hotter or milder parts of the world, especially in dry 

 exposed situations, for which their succulent leaves render them well 

 suited. About a fourth of the order inhabit the Cape of Good Hope ; 

 rather more than another fourth are found in South America ; two 

 inhabit Australia ; and the remainder are natives of Europe and other 

 parts of the world. 



Clatjtonia 



I , calyx, stamens, and pistil; 2, capsule, with the two permanent icpals ; 

 3, .\ traiHTerjc wcllon of a capsule j 4, a vertical section of a seed. 



tVvnmon Purslane (Portulaca oleracea), a potherb, now disused, 

 indicates _the harmless quality of such plants. The Talinums and 

 Calandrinias, and a few species of Portulaca, are well known as gay 

 garden-flowers. 



PORTUMMJ3 (Leach), a genug of Brachyurous Cruttacea. It 

 h the following characters : External antennae setaceous, very 



short, having their first two joints larger than the others, inserted at 

 the internal canthus of the eyes. External jaw-feet having the third 

 joint of their internal branch elongated, nearly conical, and notched 

 internally. First pair of feet large, equal, with the fingers of their 

 pincers rather long; fifth pair of feet terminated by a flattened 

 foliaceous and nearly lanceolate joint. Carapace rather flat above, 

 with the anterior border arched and semicircular, and the posterior 

 border nearly truncated, having its longitudinal diameter equal to the 

 transversal diameter ; orbits without fissures ; eyes moderate. 



P. variegaliu may be taken as an example. It is found in the 

 Adriatic Sea, the Mediterranean, and the ocean. 



Porlumnus variegatus, male. 

 a, External Antenna ; 6, External Jaw-Foot ; c, Tall or Abdomen. 



It appears to be identical with Platyonychus latipei of Milne- 

 Edwards ; nor does there appear to be any sufficient reason for 

 distinguishing the form generically from Plalyonychus, [PoRTU- 



PORTU'KID^E, a family of Brachyurous Crustacea, nearly allied 

 to the Cancerians. The species are generally known by the name of 

 Paddling Crabs. 



The general form of the Porlunidos does not ordinarily differ much, 

 from that of the greater part of the Cancerians ; but the carapace is 

 always but little elevated, and sometimes has a lozenge-shape. The 

 orbits are directed upwards and forwards. The internal antenna are 

 bent back transversely, or at least very obliquely outwards, and the 

 baailary joint of the external antenna; is partially lodged in a gap of 

 the internal orbitary angle. The third joint of the external jaw-feet 

 is always wider than it is long, and sharply truncated or notcheda 

 ita anterior and external angle for the insertion of the fourth joint. 



Orbits, Antennop, Jaw-Foot, &e., of Fortunui. 



The sternal plastron is always very wide, and in general the last 

 thoracic segment is much more developed than all the others, even 

 thau that which carries the anterior feet ; the suture which separates 

 this segment from the preceding is directed very obliquely forwards 

 and inwards ; the vault of the sides is generally nearly horizontal, 

 and the posterior sella turcica very narrow. The anterior feet are in 

 general very elongated, the succeeding feet are somewhat natatory, 

 and the posterior feet always so, their tarsus being lamellar; the 

 second pair of feet are ordinarily more than once and a half as long as 

 the carapace. 



The Crustaceans of this group are for the most part essentially 

 swimmers, and live often out at sea. They are vulgarly called 

 Paddlers. The following genera are arranged under the tribe by 

 M. Milne-Edwards : 



Carcinui (Leach). Carapace approaching in general form that of 

 Panopeus, but little convex, although sufficiently elevated, and 

 remarkably wider than it is long. The latero-anterior borders, which 

 are deeply dentated, form with the orbital border a regular curvature, 

 which does not reach beyond the level of the middle of the genital 

 region; latero-posterior borders very long and moderately oblique. 

 The branchial regions very much developed and rounded anteriorly. 

 Front advanced, horizontal, of moderate width. Orbits oval and 

 directed forwards ; there is a fissure at their upper border and one at 

 their lower border; the gap at their internal angle lodges the base of 

 the external antenna, the first joint of which is narrow and cylin- 

 drical, and reaches to the front ; their moveable stem is very long, and 

 inserted in the orbital gap. The internal anteunn are bent back in 



