120 



CONCHOLOGY 



Sometimes the name of disc (discus] is given to the 

 base of shells, as in the Haliotldea, but it is then 

 only the last turn of the spire which is comprehended 

 under that name. 



In a certain number of shells, the body part is not 

 inclined in any direction, either to the left or right, 

 forward or backward, and it is not at all excavated ; 

 the shell is then truly a flat shell, symmetrical in the 

 Sepia, &c., and not symmetrical as in Pate/la Sinensis. 

 It frequently occurs that the base and the summit 

 are united by a portion not curved in any direction, 

 but more or less excavated. These we have already 

 designated as shells covering the body of the animal 

 altogether. This occurs in the Patella, Emarginula, 

 and particularly in the Dentalium ; most frequently, 

 however, the body of the shell is formed by its con- 

 volutions in their various directions, whence they are 

 properly called spirivalve (spirivalvee). To illustrate 

 this familiarly, suppose all univalve shells to be of a 

 conical form, more or less elongated, the Dentalium, 

 for instance, and imagine it flexible ; if this be rolled 

 up, or curved from the posterior part forward, and 

 I'rorn the top to the bottom, positively in the same 

 vertical plane, the result would be a discoid shell, 

 compressed from the right to the left, the summit of 

 which can only be visible in the same direction, and 

 the axis of which is everywhere equally transversal. 

 Such shells may be called rolled up (revolutce). An 

 example strictly in point may be found in the 

 Argonauta and its congeners ; but the Planorbes, 

 though in some respects analogous, are in fact only 

 sub-revolutce. 



The principal distinctions, and the various modes 

 in which shells are rolled, consist in their greater or 

 less degree of perfection. They are called arched 

 (arcuata) when the shell presents only a slight curve, 

 as in certain species of lielemnite, and as in the 

 Dentalium. Curved (curvata), as in the Ammonocera- 

 tiles, the shell presenting a much more circular curve. 



Half-coiled (semi^revolttta), when the shell is so 

 curved as that the turns of the spire do not touch, 

 which occurs in the Spirula, presenting a spiral coil 

 on a plane. 



Coiled (revoluta), when the turns touch each other, 

 as is the case with the true Ammonites (per-revolutcc'), 

 when the spiral turns penetrate each other in such a 

 way that the last conceals all the others ; which is 

 illustrated by the Nautilus. 



If, on the contrary, the revolutions of the spiral 

 cone are formed tranversely from left to right, as 

 placed on the animal, such shells are termed invo- 

 luted (involute), or, as Linnaeus calls them, convoluted, 

 convoluted. In these species the base is nearly the 

 whole length of the shell, as well as the aperture, and 

 the axis of revolution is necessarily longitudinal. 

 In fact, shells are scarcely ever completely involuted. 

 Those which the nearest approximate that form are 

 the Ovulae and Cyprece. Sometimes the shell does 

 not form a complete turn, as in the Sulla, and then 

 the aperture is as large and as long as the shell itself. 



The greater proportion of univalve shells are, 

 however, intermediate with the two above-described 

 formations ; that is to say, the body of the shell is the 

 result of an oblique revolution from right to left, and 

 from the lower to the upper part, if we proceed from 

 the bottom to the top ; or, on the contrary, which is 

 more natural, if we view the shell according to its 

 progress of growth. Such are the true spiri-valve, 

 or turbinated shells. 



The name of spire (clavicula) is given to all that 

 portion of the shell which is formed by the revolution 

 of the spiral cone. Sometimes the last turn of the 

 spire, which is usually the largest, is distinguished by 

 the name of body (corpus) ; the part corresponding 

 with the aperture is the belly (venter) ; that which is 

 opposed to it is the back (dorsuni) : to all the other 

 parts of the shell the term clavicula applies. By far 

 the greater number of molluscs terminate on the 

 right side of the animal ; some, however, which we 

 have pointed out, terminate to the left ; and this 

 circumstance distinguishes the sinister from the dex- 

 tral shells, dextrae and sinistrorsce. 



Having thus pointed out some of the leading 

 distinctions which characterise the plan of formation 

 presented by shells, we shall briefly enumerate the 

 scientific terms applied to the position of the shell 

 with regard to its inhabitant, leaving the general 

 nomenclature to an alphabetical arrangement. 



Dorsal (dorsalis) shells are such as are placed on 

 the back of the animal. Of these there are only 

 examples to be met with in the univalves and sub- 

 bivalves ; and in the Chiton among the multivalves. 



Ventral (ventralis) applies to shells placed beneath 

 the animal's belly. The genus Ombrella may be 

 supposed so situated. 



Dorso-ventral (dorso-vcntralis) are shells composed 

 of two pieces, the one on the back, the other beneath 

 the belly, as in the Terebratitla, Lingula, &c., and 

 others of the class Palliobranchiata. 



Bilateral (bilateralis) are those of which the com- 

 ponent parts are, one on the right, and the other on 

 the left of the animal, as in the Ostrea and its con- 

 geners, which occurs in all the class of the Lamelli- 

 branchiata. 



Perisomatic (perisomatica), when the pieces forming 

 the shell beyond the number of two, surround the 

 body of the animal, as occurs in the Balanus, Anatifa, 

 and their congeners, constituting the class Nenia- 

 topodes. 



External or internal shells have been so called from 

 anatomical investigation of their position with regard 

 to the animal. External is applied to nearly all 

 shells which cover the skin of the animal, and it is so 

 obvious, that no further explanation is requisite. 



Internal shells are those in contradistinction to 

 the generality, which are partially covered by some 

 portion of the animal's skin, more or less thick. The 

 shell is then, for the most part, very thin, flat, or only 

 slightly revolved. It is constantly of a pale colour, 

 and does not ever possess an epidermis. No ex- 

 amples of these are found out of the order Poly- 

 thalamacea, or many chambered shells, as also of the 

 single chambered, or Monothalamacea order. Cer- 

 tain species of Mya may also be presumed so situated 

 as to be properly termed internal shells. 



The terms applicable to the structure of shells 

 depend upon the solidity and arrangement of matter 

 composing them. Thus shells are called solid (solida) 

 when they are heavy and capable of resisting con- 

 siderable violence, such as the Mitres and Olives in 

 univalves, and the Spondylus, Crassatina, Pectina, 

 and other of the bivalves ; fragile (fragilis), when on 

 the contrary. 



Thick (crassa), generally understood to apply to 

 the valves rather than to the whole shell. 



Thin, papyraceous (tenuis, papyracea), when the 

 shell is excessively thin. 



Transparent (translucida), when such thin shells 



