M O L L U S C A. 



629 



lusca. merco arc procured. The Pinna, so famous, for fur- 

 V"^ nishing a byssus or kind of thread, with which gar- 

 ments can be manufactured, likewise produces pearls 

 of C" : e size. They have seldom the silvery 



whiteness of the pearls from the Aviculn, being usual- 

 ly tinged with brown. But the shell which in Britain 

 produces the finest pearls, is the L'liio margarilifera, 

 which, as placed by Linnaeus in the genus .V/'i. It 

 is found in all our alpine river?. The Conwuy and 

 the Irt in England, the rivers of Tyrone and Donegal 

 in Ireland, and the Tay and the Vthan in Scotland, 

 have long b.-en famous for the production of pearls. 

 These concretions are found between the membranes 

 of the cloak of the animal, as in the Avicula. or adher- 

 ing to the inside of the shell, as in the Unio. In the 

 former case, they seem to be a morbid secretion of tes- 

 taceous matter ; in the latter, the matter appears to be 

 accumulated against the internal opening of some hole 

 with which the shell has been pierced by some of its 

 foes. Linmrus, from the consideration of this circum- 

 stance, endeavoured, by piercing the shell, to excite 

 the animal to secrete pearl ; but his attempts, though 

 they procured him a place among the Swedish nobility, 

 ana a pecuniary reward, were finally abandoned ; the 

 process being found too tedious and uncertain to be of 

 any public utility. The largest pearl of which we 

 have any notice, is one which came from Panama, and 

 was presented to Philip II. of Spain, in 1579- I< was 

 of the size of a pigeon's egg. Sir Robert Sibbald men- 

 tions hi* having seen pearls from the rivers of Scotland 

 as large as a bean. 



AU the animals of this class inhabit the waters. A 

 few species live in pools, lakes, and riven, preferring 

 fresh water ; but the sea contains the greatest numbers. 

 The following arrangement of the genera is the one 

 adopted by M. Cuvter in his " liegne Animal." As it 

 stands at present, it brings together into groups near* 

 !y allied genera, without, however, distributing them, 

 with much accuracy, into natural families. 



SECTION I. 



In the C oncliifera which are brought together in this 

 section, the cloak is open, and the water comes direct- 

 ly in contact with the gilU and mouth. The foot, when 

 it exists, is very small ; and, instead of aiding locomo- 

 tion, appears to be principally employed in forming 

 and fixing the threads of the byssua. Many of the 

 species are cemented u the rocks from iu fancy to old 

 age, others are capable only ( Suiting to a short dis- 

 tance, by violent exertions, consisting in suddenly 

 opening and shutting their valves. 



I. 1 :;.. k_ The animal* of this tribe are distinguish- 

 ed by possessing only one adductor muscle for closing 

 the valves of the shells, which are inequivalve. The 

 distribution of the genera into natural families, is at 

 present impracticable, a* the animals of few of the 

 species have been investigated with any degree of 

 care. 



In the genus OSTHEA. represented by the common 

 oyster, the abdomen is prominent, the adductor muscle 

 cmtial, and the branchiic united at their summits with 

 the edge of the clock. The animals of the genera 

 Acardo and Gryphrea, are unknown. 



In the animal* of the genus Pet-ten, represented by 

 the common scallop, t'.ere is a small foot, supported 

 on a short stalk arising from the abdomen. The mar- 

 gin of the clo.i . muled with two rows of ten- 

 taruU, son) c of wliic'i, in the external row, have green- 

 ish tubercu'-aUd -nmimu Thr mouth is surrounded 

 with numerous branched u-uucula, in place of the four 



ordinary labial appendages. The genera Lima and Mollusca. 

 Pedum are nearly related to the genus Pecten, with ' 

 which they constitute a particular tiunily. 



The genus ANOMIA, as now restricted, is distin- 

 guished by the singular character of the adductor mus- 

 cle, a portion of which is attached to the corneous or 

 testaceous plate, which passes through the cardinal 

 perforation, and adheres to rocks. There is a small 

 foot, which is capable of being likewise protruded 

 through the cardinal perforation. 



In the genus SPONDYLUS, the margin of the cloak is 

 fringed with a djuble row of tenlacula, with tubercu- 

 lated f.inmiits, and the foot is seated on a short stalk, 

 with a large radiated disk. 



Besides the genera already noticed, the Placuna, 

 Plicntula, Malleus, Vulsella, and Perna, likewise belong 

 to this tribe. Many of the species are eagerly sought 

 after as food, as the oyster and scallop, and give em- 

 ployment to several thousands of the inhabitants of 

 this country, in fishing and conveying them to the 

 market. 



In the revolutions which molluscous animals have 

 undergone, the genera of this tribe appear to have suf- 

 fered greatly. Multitudes of extinct species, and even 

 genera, are to be found in the solid strata, differing in 

 shape and size from those which are now found in our 

 seas. We are disposed to refer to this extinction of 

 genera, as accounting for the circumstance, that in this 

 tribe almost every genus stands apart, and appears to 

 be but remotely connected with those with which it is 

 now associated, the connecting links having been de- 

 stroyed. 



II. TRIBE. The valves in this tribe are moved by 

 means of two adductor muscles, one of which is plac- 

 ed near the head, and the other at the anus of the 

 animaL 



The pearls produced by the animals of the genus 

 AVKULA, have been long known and highly prized 

 but their anatomy and physiology are still involved in 

 obscurity. The ./. hirundo has been determined by 

 Mr. Sowerby to be a native of the British seas, he 

 having obtained specimens both from Marazion and 

 li.mtry Hay. In the genus Crenatula of Lamark, which 

 \- intimately connected with the preceding, there is no 

 mark indicating the animal to have a byssus, which the 

 Avicula is known to possess. 



The genus PINNA has been long known and highly 

 prised by collectors. The mouth of the animal is si- 

 tuated near the narrow end of the shell. Near the 

 anus there is a conical appendage, susceptible of en- 

 largement and contraction, who^e use is unknown. 

 The thread or bysMis with which the animal adheres 

 to the sand in which it resides, is collected on the 

 Neapolitan coast ; and when mixed with silk, is woven 

 into various articles of dress, as gloves and stockings. 



The genera Area, Pectuuculus, and N'ucula, form a 

 . formerly included in the genus Area of Lin- 

 MM. The animals of the genus Area have a pedal li- 

 gament, with which they adhere to different bodies. 

 The animal of the Pectunculus, on the other hand, is 

 I'urimhed with a large compressed foot, with which it 

 is able to crawl. The inhabitant of Nucula is un- 

 known 



SECTION II. 



The animals of this section have two adductor 

 muscles. The cloak is open in front, with a separate 

 aperture behind, for the passage of the excrements. 

 'The shells are equivalve. 'I he species were formerly 

 included in the genus Mytilus. 



