CONCHOLOGY. 



121 



permit of the rays of light to pass partially through 

 them, as is the case with the Carinaria, Anatina, &c. 



Lamellar (lamellosa) or foliaceous (foliacea), when 

 the strata of matter are not compact and solid, par- 

 ticularly towards the edge of the shell, as in the Oyster. 



Fibrous ( fibrosa), when the shell fractures in a per- 

 pendicular direction, from its summit to the base, as 

 in the Pinna. 



Nudu, or naked, as in shells whose surfaces appear 

 highly varnished : the Olives, Cowries, &c. 



Corticated (corticata). or epidermata, when the 

 edges (of the stria of increased growth) form on the 

 surface of the shell, an envelope more or less thick, 

 the appearance of which occasions their being distin- 

 guished as furry or scaly. 



The chemical composition of shells separates the 

 species into such as are sandy (arenacete), in which 

 the calcareous molecules are not united with each other, 

 so that they may easily be reduced to small portions 

 or grains, as in the grey snail. 



Cretaceous (cretacece) or chalky, when the super- 

 abundance of calcareous matter, renders the shell 

 extremely friable, as in the Tubicola. 



Horny or membranaceous (membranosce') when, on 

 the contrary, the mucous matter constitutes nearly the 

 whole of their substance, as in the Apfysia. 



The colours of shells, as of everything else, does 

 not admit of any misunderstanding, and must be ap- 

 plied according to the ideas each one forms of their 

 degree of intensity. They are infinite in their com- 

 binations, though not a very important consideration, 

 as they do not affect any peculiar arrangement ; they 

 may, therefore, be noticed under two heads. Super- 

 ficial (snperficialis] as in shells, externally coloured, as 

 most species are ; or imbued (imbvtui) as in shells, 

 where the colouring matter penetrates the substance, 

 such as the Janthina and others. 



The system of coloration, or as it is more familiarly 

 termed, the marking of shells, gives rise to the follow- 

 ing generally used distinctions. It is uniform (uni- 

 formis) when equally distributed over the whole 

 surface of the shell ; or variegated (variegala), on the 

 contrary, when the colouring matter is variously ar- 

 ranged, and these differences of marking are called 

 fasctated, ribboned (fasciata], when the zones or bands 

 are of a different colour from the groundwork ; it may 

 be fasciated longitudinally, when they run from the 

 summit to the base, folio wing the direction of the whole 

 of the spire. In bivalves this is distinguished by 

 radiated or rayed (radiata) ; or, if in the direction of 

 the stria', which mark the shell's growth, they are 

 termed transverse (transversalis) ; lineated (lineata) 

 when the coloured bands are closely placed, like hair 

 lines, as in the B ulla physa ; lettered (scripta), when 

 the bands are more or less waved or broken, resem- 

 bling Arabic characters, as occurs in some of the 

 volutes. Tesselated (tettelata), when the system of 

 colouration presents large spots.or other shaped marks, 

 comparable to inlaid marble or mosaic work. When 

 these markings are round, or nearly so, they are 

 called spotted (maculatd), and when the spots are dis- 

 posed in bands, the shell is said to be ribbon-spotted 

 (fascio-macnlata); when the spots are very small, re- 

 sembling rly-marks, they term it dotted (punctata), 

 and fatcio-puttctata, when these minute dots are dis- 

 posed in bands. 



Of the general form of Univalve and Sub-Bivalve 

 Shells. In considering the general form of Univalve 

 shells, without paying attention to the distinction of 

 their uarts, certain terms are used which, although 



rather vague, must necessarily be known. The first 

 distinction regards the equality or inequality of the 

 two sides of a shell of any form whatever, separated 

 by a fictitious axis drawn from the summit to the base, 

 or from one extremity to the other. A symmetrical 

 shell is one whose valves are perfectly equal, and a 

 non-symmetrical shell, such as has its valves unequal, 

 thus the shell of the Argonauta, the Lippet, &c., are 

 symmetrical ; the Sigaretus, and many others, are non- 

 symmetrical. 



A flat shell (plana,) is that which has no cavity ; 

 tubular (tubulosa), that of which the diameter is con- 

 siderably less than its length, as in the Dentalium ; 

 recovering (aperient), that which is conical, and with- 

 out a spire, properly so called, and so placed on the 

 animal as to be easily taken off, as with the Limpet ; 

 spiral (spiralis], that which is more or less twisted in 

 various fashions, as we shall more fully explain here- 

 after ; but for the moment we will continue to define 

 some terms belonging to the shell examined as a 

 whole. It is called discoid (discoided), when it resem- 

 bles a disc, and which, considering the manner in 

 which the spire is evolved, we call rolled up, as occurs 

 in the Ammonites; depressed (depressd], such as are 

 oval, or rounded very flat, the spire very short, as the 

 Sigaretus. The same name is sometimes employed to 

 designate certain shells whose last whorl, or the body 

 of the shell, enlarged by lateral bands, appears flat- 

 tened from the top to the bottom, as in the Ranella ; 

 globular (globosa or ampullacea), that in which the 

 diameters are nearly equal, occasioned by the great 

 development of the last whorl of the spire, which is 

 much larger than that which preceded it, as in the 

 Ampullaria, &c. ; oval (ovalis), when the longi- 

 tudinal diameter is rather longer than the trans- 

 verse, as in the Cowries, Olives, and many others ; 

 bordered (marginata), when the edges are of a greaer 

 thickness than the rest of the body, as in some species 

 of Cowry, &c. ; navicular (navicularis), some shells 

 which, reversed on the back with the aperture up- 

 wards, have some resemblance to a little boat, as in 

 the Argonauta ; pyriform, when one of the extremities 

 is inflated and rounded, and the other pointed in the 

 form of a short tail, as in the Pyrula. 



Claviform, club-shaped (clavata), when the body of 

 the shell is short and inflated, and the anterior part 

 narrow and lengthened, as in the Murex haustellum 

 (Linnaeus). Beaked (ro$trata\ when it is terminated 

 at its two extremities by a prolongation of the form of 

 a beak, as in the Ovula birostris (Lamarck). Conical 

 (conica), when one of its enlarged extremities is as if 

 it were cut square, the other being pointed and form- 

 ing the summit; when it is the summit itself of the 

 shell which forms the summit of the cone, it is then 

 called a turbinated shell, as in the Trochus, and the 

 Turbinella ; and it is called conical or conoid, when, 

 on the contrary, the summit of the cone is at the an- 

 terior part of the aperture, as in Cones, properly so 

 called. Cylindrical (cylindrica), when the shell is 

 lengthened, and nearly of the same size throughout, 

 as occurs in most of the involuted shells, such as the 

 Olives, &c. Fusiform, spindle-shaped (fusiformis), such 

 as are swollen in the centre, and pointed more or less 

 at the two extremities, as in the Fiisus. Turriculated 

 (turriculata), those that are very much elongated, that is, 

 whose longitudinal diameter is much greater than the 

 transverse, which depends on the manner in which 

 the spire is formed, as in Turritella. After this short 

 explanation, univalve shells may be considered with 

 regard to the distinction of each of their parts. A con- 



