PLACENTULA PLANORBIS. 



475 



PLACENTULA. A minute species of mollusc, 

 considered and classed by Fichtel with his micro- 

 scopic Nautili. 



PLACOBIIANCHUS. A genus of naked mol- 

 luscs established by Hasselt from specimens disco- 

 vered in Java. It, appears by his description to differ 

 essentially from other molluscous animals ; but its 

 description requires more frequent observation of the 

 species to enable us to assign to it a proper position 

 in malacology. As far as \ve are able to judge, it 

 appears to belong properly to the Cyclobranchiata, 

 and nearly allied to the genus Doris. 



PLACUNA(Lamarck ; ANOMIA SEI.LA, Linnaeus). 

 The animal appears to be entirely undescribed of this 

 singular mollusc; but the shell id well known to ama- 

 teurs, and once bore a high price. This genus 

 immediately precedes the geuusAnomia in Lamarck's 

 system, with which it was blended by Linmcus, 

 though in form it bears but little resemblance ; unlike 

 them also in habits, it is never affixed to other bodies, 

 and the probable organisation of the animal must 

 likewise be perfectly distinct. The name given to 

 this shell by Liinituus is not quite so happy as many 

 others which have led him from their resemblance to 

 things to adapt generic names. If that alone should 

 guide the naturalist, this might better have been 

 named a saddle-cloth than a saddle. It is a most remark- 

 able mollusc, unlike every thing else in malacology, 

 and, viewed in an anatomical light, probably presents 

 more to astonish us than any other testaceous mol- 

 lusc, were we only to consider that a shell, often six 

 inches in diameter, encloses an animal by which it is 

 constructed, whose utmost solidity at any part does 

 not exceed the thickness of a sixpence, while every 

 indication of muscular power is as strongly developed 

 as in the gigantic inhabitant of the Tradacnn, or other 

 species of shells of an inferior size, such as the com- 

 mon oyster, whose valves cannot be torn asunder 

 without the application of an immense mechanical 

 force. The principal characteristic features of this 

 singular mollusc are as above stated ; the extreme 

 thinness of the animal's body, the great delicacy of 

 the texture of the valves which admit the rays of 

 light to pass through them, and the strength of the 

 hinge which presents a point cfappui, are scarcely 

 to be equalled by any other mollusc. The very 

 peculiar form of this hinge completely distinguishes 

 this genus from all others; on one valve it may be 

 described as resembling the letter V, the lower parts 

 not quite joined ; there are corresponding cavities on 

 the opposite valve, so deeply indentated, that they 

 sometimes form an external projection, the substance 

 of the shell is thin, semi-pellucid.foliaceous, and the mar- 

 gins fringed ; its form is sometimes orbicular, and very 

 liar, as in the P. papyrucea, which, from its diaphanous 

 properties, is used in Japan as a substitute for glass 

 in their windows ; the other species is generally of a 

 squarer form, and singularly bent, as if partially 

 folded or warped over a cylinder; the valves of both 

 species are finely striated or wrinkled longitudinally, 

 and the ligament attached to the powerfully-con- 

 structed hinge, the interior presents a muscular im- 

 pression similar to and as large as that of the oyster. 

 The Placuna sella presents a slight tinge of purple in 

 the colour of its valves ; but the P. papyracea is 

 nearly white. There are three recent species known as 

 inhabiting the Indian seas and the coasts and islands 

 of Japan, which abundantly furnish the P. papyracea. 

 In a fossil state they have been discovered, but they are 



rare. A modern naturalist asserts, that the muscular 

 impression of this shell is not remarkable ; but surely 

 he cannot have examined many specimens; \ve only 

 refer our readers, who are interested in reconciling 

 what may be termed anomalies in nature, to such ex- 

 amples as may daily be seen in the British Museum, 

 in which we assert, without the fear of contradiction 

 by any one acquainted with nature's indications of 

 animal strength as exhibited in malacology, more 

 strength is indicated as belonging to this feeble fragile 

 shell than is marked in testaceous molluscs, whose 

 habitations would bid defiance to external injury ; and 

 from their size and substance require a mechanical 

 force incalculably greater than appear requisite for 

 the shells of this genus ; and the conclusion naturally 

 is, that they are furnished with a closing muscular 

 apparatus sufficiently powerful to crush their own 

 valves, and equal to that necessary for retaining 

 closed, or shutting the valves of molluscs incom- 

 parably thicker. 



PLAGIOSTOMA (Lamarck). As no recent 

 example of this mollusc has been discovered, Lamarck 

 assigns to it a supposititious place in the connecting 

 chain of malacology, and he appears to consider it 

 an intermediate link between the genera Lima, Pec- 

 tor, Spondylus, and Podopsis. The shell is sub-equi- 

 valve, free, sub-auriculated, and the cardinal base 

 straight and transverse, the apices are but little 

 distant; the external edges of the hinge angular, 

 straight on one side, oblique on the other, the hinge 

 not exhibiting any teeth or serrations ; there is a 

 cardinal cavity between the apices, partly internal, 

 opening outwards, and receiving the ligament. The 

 animal is of course entirely unknown, and illustrates 

 one of the numerous instances of animated beings who 

 have totally disappeared from the face of nature, 

 leaving the naturalists to draw analogous conclusions 

 in support of their rank in the classification of recent 

 objects of nature's formation. De France enumerates 

 three species, but more have been discovered since 

 his excellent work on Fossil Conchology. 



PL AN AXIS (Lamarck; BCCCINUM SULCATUM, 

 Born). These are marine shells, nearly allied to the 

 genus P/i(iM(inel/a,but the animal is entirely unknown. 

 A distinguishing character from that genus is mani- 

 fest, the Plana.vis having the base of the columella 

 truncated as in the Hfclauojjsis, and it would have 

 been still stronger, if, as Lamarck conjectured to be 

 the case, they had not possessed an operculum. 

 Shells of this genus are very small in general, trans- 

 versely grooved on the exterior, of an oval conical 

 form, solid, the aperture ovate, columella flattened, 

 truncated at the base, and separated on the right 

 margin by a narrow sinus or channel ; the interior 

 surface of the right margin is grooved or rayed with 

 a callosity running beneath the summit, appearing in 

 that respect allied to the genera Buccinum and Pur- 

 pura. Sovverby, in his Genera of Shells, observes 

 that he has seen one specimen with a thin horny 

 operculum (a circumstance unknown to Lamarck), and 

 he considers them more nearly allied to the Bucc'ma 

 and PurpitrcB than to the Phasianelte, which possess 

 a solid calcareous operculum, and also by their having 

 a callosity running under the summit of their right 

 lip. Only two species appear to be known, one from 

 India and the other from South America. 



PLANORBIS (Lamarck; HELIX, Linnaeus). 

 M tiller and Bruguiere were the first naturalists who 

 separated this mollusc from the genus Helix of Liu- 



