SHELL-INSECTS. 



SIBTHORPIA. 





growth of the animal, the second layer of shell, au<l afterwards the 

 u>nerm<Mt layer, U deported up to the edga nf the mouth, which is 

 thui completed. By thu proem a spiral ibell, a ooue for instance, 

 become* a* it wen rolled on itself. 



But besides this gradual onward increase, another process is called 

 into action in certain cases, in order to the symmetrical growth of the 

 spiral shell. Take a Triton or a Mwrtx for instance : the intervals of 

 growth are marked by the elevated varices. In the direction in which 

 the shell is, so to speak, to be rolled on itself, there will be found a 

 thickened spreading inner lip, into which a rarix descends. Now 

 both these must be removed, in order to insure the uninterrupted 

 progress of the shell ; for if these shells bo examined internally, they 

 will be found to be smooth and uninterrupted, and what is now inside 

 was once outside. The power of absorption then must be great to 

 remove the masses of abell which are contained in the highly-elevated 

 and wrought varice, for such in many shells they are, to say nothing 

 of the thickened and patnlou* lip. 



In the smooth shells, cones for instance, where the animal has no 

 obstacle to remove in the enlargement of the shell, the internal 

 volutions are often so much absorbed as to be as thin as paper, and 

 not unfrcquently have vanished altogether. In both these cases, may 

 not the animal derive from its own shell the means of increasing it ? 



The Operculum would in itself almost justify a treatise, which our 

 limit* forbid. It is a horny or shelly plate, which closes more or less 

 completely the aperture of the spiral shell ; so that those spiral shells 

 which are so furnished are in fact bivalves. They are developed with 

 the embryo in the egg. We must refer our readers for details to the 

 interesting papers of Dr. Gray, who has for some years past paid 

 great attention to this subject In the ' Medical Repository for 1821 

 he first urged the importance of this adjunct as a generic and family 

 distinction, and he pursues the subject in the memoir from which we 

 have quoted in this article ; nor baa the study of this interesting part 

 of the conformation of many testaceous molluscs been neglected by 

 U. He Blainville and other French zoologists, who have followed it 

 out with great assiduity. 



The nucleus, spire, and indeed the upper whorls of many shells, of 

 the Volutes for instance, and most especially of Uayilut, become, as 

 they advance in growth, filled more or less by a transparent calcareous 

 secretion ; and Dr. Gray remarks that the distinction between such 

 shells and those which are decollated, such as SiUinut decollate, 

 Ccriikium dtcoUalum, &<x, is, that in the latter the animal, instead of 

 lining the upper whorls with an internal coat, suddenly withdraws its 

 body from them, and forms behind its extremity a concave septum : 

 the vital communication between the body and the apex of the shell 

 being thus cut off, the latter part, be observes, decays in the manner 

 of a dead shell, and falls off in particles ; but M. De Blainville refers 

 the decollation of the spire to the filling up of the inner surface of the 

 cavity of the shell with a very brittle vitreous deposit 



Dr. Gray remarks that in many fresh-water bivalves there is depo- 

 sited between the layers of the shell a lamina of animal matter, 

 itimilar to the periostracum ; and that in the genera Etheria and 

 Mulleria such a coat is deposited between nearly all the layers, giving 

 them a very peculiar olive-green colour, and having minute dots on its 

 surface. The erosion of these sheila must be familiar to all who are 

 conrenant with the subject ; and Dr. Gray states that these successive 

 depositions of animal matter enable them to offer a new layer of 

 periostracum for the protection of each succeeding plate, aa the plate 

 above it yields to the destructive influence of the medium in which 

 the animal resides. A similar deposit of animal matter is also found, 

 be addn, forming green stains in the pearly inner coat of the various 

 specie* of L'nimet, and protects from the action of the water the inner 

 part of the umlonea of shells which have been eroded. Dr. Gray has 

 observed the thick iuner layer in the upper valve ofOitrea Cornucopia 

 to be prismatic, and the outer part of the lamina) to be separated by 

 layers of periostracum. 



The disintegrated thin, lamellar, pearly gray, silvery scales of 

 iridescent shells, when reduced to powder, may be used as a pigment 

 to imitate the silvery appearance of fishes, Ac., and indeed the disin- 

 tegrated and powdered scale* of the 1'lacuntt (Moon-Shells, Chinese- 

 \\ in' low Oysters, as they are called by collectors), are so used by tho 

 Chinese in their water-colour drawings. 



The term Shell is also commonly applied to the covering of crunU- 

 ceoiiK animals and the crusts of Echini : thus people familiarly talk of 

 the shell of a lobster or crab and of the shell of a sea-egg. Mr. 

 Hatehett remarks that there is reason to conclude that phosphate of 

 lime mingled with the carbonate is a chemical characteristic which 

 distinguishc* the crustaceous from the testaceous substance ; and that 

 the principal difference of the qualities of each, when complete, is 

 caused by the proportion of the hardening substances, relative to the 

 gluten by which they are cemented ; or by the abundance and con- 

 aistency of gelatinous, membranaceoua, or cartilaginous substance, in 

 and on which the carbonate of lime, or the mixture of carbonate and 

 phonphate of lime, has been recreted and deposited. Thus the presence 

 of phosphate of lime mingled with carbonate appear* to be a chemical 

 character of Crustaceans and AV/imi. 



MIKI.I/.INSKGT& [ESTOMOSTRAIA.] 



Sll K-UAK. (fAM-ABIKACE*.] 



SHEPHAKDITK. [MAOKBHA.] 



SlIK.rilKHD'S NEEDLE. [Soismx.] 



SHEPHERD'S I'l.-KSB. [CAP8EIAA.] 



SHKKAKDIA (so named by Dillenius after his patron William 

 Sherard, LL.D., consul at Smyrna), a genus of Plants belonging to tho 

 natural order Jtukiacete. It has a funnel-shaped corolla, a dry fruit, 

 crowned with the limb of the calyx, which in C-toothed. 



& arreniit, Corn-Kield Madder, is the only species. The stem i* 

 mostly decumbent, branched, square, and leafy ; the whole plant is 

 rough and hairy ; the leaves are six in a whorl, acute, and obovnto 

 lanceolate ; the flower is blue, in a small sessile terminal umbel. It is 

 found on sandy soils, in Great Britain, Europe, and the Crimea. 



(Babington, Manual of Jiritiih llatnny.) 



SHO'REA, a small Indian genus of Plants, belonging to the natural 

 order IHpterocarpaceat, named in compliment to Sir J. Shore, afterward* 

 Lord Teignmouth, then governor-general of Bengal. The genus is 

 found as far south as the Tine ; and & roltutta, the best known and 

 most useful species, as far north as 30 N. hit, in many parts forming the 

 forests which skirt the south-western base of the Himalaya Mountains. 



The genus is characterised by having a calyx of 5 sepals eulaiging 

 into 5 long wings ; petals 5 ; stamens 25 to 30 ; fruit 1 -celled, 3-valved, 

 and 1-seedod. 



The family to which the Shorta belongs is remarkable for tho 

 number of useful products yielded by its different species, as the 

 camphor of Sumatra, resin, wood-oil, and valuable timber. >S. roAwrfa 

 is remarkable on all these accounts, aa it is a lofty and ornamental 

 tree with showy inflorescence. It is well known as a timber-tree by 

 the name of ' Saul,' or ' Sal,' and chiefly employed in the north- 

 western provinces of India in all government works, house-timbers, 

 gun-carriages, &c. The wood is of a uniform light-brown colour, 

 close-grained and strong. The tree exudes a resin which by the natives 

 is called ' ral,' and by the Europeans one of the kinds of Dammcr, 

 being used for the same purposes as many other resins, and in Bengal 

 very frequently as a substitute for pitch in the dockyards. It is also 

 sometimes used by the Hindoos as an incense. 



SHRIKES. [LANIADJJ.] 



SHRIMPS. [CRANGONID.B.] 



SHRIMPS, FRESH-WATER. [GAMiuncs.] 



SHRUB (' Frutex'). All plants are divided into herbs, shrubs, and 

 trees. A shrub is a plant with a woody stem, which approaches the 

 tree in its duration and consistence, but never attains the height of a 

 tree, and is generally taller than the herb. It varies in height from 

 about four to twelve feet Linmeus attempted to distinguish tree* 

 from shrubs by the former having buda and the latter having none. 

 But this distinction is of no avail, aa plants like the willow, generally 

 called shrubs, possess buds, whilst most trees in hot countries are 

 without them. 



In horticulture shrubs are too well known to need a definition. For 

 practical purposes they are divided into the deciduous and evergreen 

 kinds, and each of these kinds may bo further divided, according to 

 their hardy or tender habits, their form, size, mode of growth, colour, 

 &c. The most ornamental flowering shrubs ore those belonging to the 

 genera Roia, Rhododendron, Azalea, A'a/mia, Andromeda, raccini'um, 

 <to. Among the evergreen shrubs are the Holly, the Ivy, the Jasmine, 

 the Box, various Heaths, &e. 



Shrubs are often planted together, forming what are called shrub- 

 beries, and when the kinds are judiciously selected and arranged, 

 these collections add greatly to the beauty of the gardens and pleasure- 

 grounds where they are introduced, 



S1BBALD1A (in honour of Robert Sibbald, formerly Professor of 

 Physic at Edinburgh), a genus of Alpine Plants, belonging to the 

 natural order Rotacea. It has a concave 10- ported calyx, the 5 outer 

 segments accessory ; it has fi yellow or white petals, 5 sepals, and a 

 lateral style; the fruit consists of from 5 to 10 small nuts seated on a 

 dry receptacle. 



X. procumbent has trifoliate leaves, wedge-shaped leaflets, with 

 three teeth at the apex, rather pilose, the flowers corymbose ; petals 

 yellow, small, shorter than the calyx. It is found on dry mountains 

 in Scotland, in Europe, Siberia, and North America. 



fi. parviflon is a native of Cappadocia. It has trifoliate leaves, the 

 leaflets beset with strigose pili on both surfaces, the flowers in glomerate 

 heads, the petals obovate, one-half shorter than the calyx. 



(Kabiugton, Manual of liritiik Botany.) 



SIUTHO'RPIA, a genus of Plants named after Dr. Sibthorp, pro- 

 fessor of botany at Oxford, the successor of Dillenius, and the father 

 of Dr. John Sibthorp, the traveller. This genus consists of small 

 creeping, rooting, tufted herbs with small alternate uniform leaves, a 

 5-parted calyx, 6-lobed subrotate corolla, 4-5 nearly equal stamens, a 

 capitate stigma, and a suborbicular capsule dehiscing at the top. Tho 

 flowers are axillary, solitary, and inconspicuous. One of the species, 

 S. Knropfra, is a native of Europe, and is found in Portugal, Spain, 

 and France, and in Devonshire and Cornwall in England. This genus 

 is referred by most botanists to the order teropkulariacca: ; but it 

 differs from this order in its globose placenta and regular flowers. It 

 differs from Primulacea in its stamens, being alternate with the lobes 

 of the corolla, and in its capsule being 2-celled ; hence Don has pro- 

 posed to place this and some other doubtful genera in a new order, 

 i*ilMor;iiace<r, which possesses characters intermediate between those 

 of Sci-o; hulariacea and Frimulacea, 



