MOLLUSCOUS ANIMALS. 



583 



ovarium, ffgy; the byssus, h i; the abdomen, k; s, the 

 anal opening. 



FAMILY II. MYTILACEA. 



Hinge with a sub-interior ligament, which is marginal, 

 linear, and very entire, occupying a great part of the 

 anterior border; shell but seldom foliaceous. Nearly 

 the whole of this family attach themselves to marine 

 bodies by a byssus. They have a tongue-shaped foot, by 

 the aid of which they attach their byssus to extraneous 

 substances. Pinna nobilis, pi. 24, f. 91 . a, b, c, d, lobes of 

 the mantle: p .<?, h Ti, adductor muscles: * f, part of the 

 abdomen. Mjt&i edulis, pi. 24, f. 7 1 . f. 76, the animal. 



FAMILY III. TRIDACNITES. 



The shell is transversely equivalve, with the muscular 

 impressions situated under the middle of the upper 

 margin, and prolonged on each side; the mantle is close 

 and perforated by three holes, through one of which 

 passes the foot ; the second furnishes an entrance and 

 .exit for the water requisite for respiration; and the 

 third is a passage for the excretions; the two latter 

 not being prolonged into tubes as in the Mytilacea 

 Tridacna ffiyas, pi. 24, f. 64. The tendinous byssus by 

 which these gigantic shells sometimes weighing 300 

 pounds attach themselves to rocks, is so thick and 

 strong that an axe is required to sever it. Nearly allied 

 to this species is Tridacna squamosa, pi. 32, f. 27. 



ORDER II. DIMYAIRA. 



Shell having two distinct and lateral muscular im- 

 pressions, which are deep and widely separated, being 

 placed near the lateral extremities of the valves. 



SECTION I. 



Shell always inequivalve and irregular. 

 FAMILY I. CHAMACEA. 



Shell inequivalve, irregular, and adherent; with two 

 separate muscular impressions; ligament placed exter- 

 nally ; the animal provided with short, separate siphons, 

 C/iama f/ryptioides, pi. 24, f. .54. Ckama atttiquata, pi. 24. 

 f. 55. a 6, the lobes of the mantle; c, the abdomen; d d, 

 the foot. 



SECTION II. LAMELLIPEDES. 

 FAMILY I. NAYADES. 



This family consists of fresh water shells; the hinge 

 sometimes provided with an irregular, simple, or cleft 

 tooth, and a longitudinal prolonged one; sometimes 

 destitute of any; or is furnished in its whole length with 

 irregular granular tubercles ; the posterior muscular im- 

 pression compound; and the umbones often decorti- 

 cated; the muscular impressions are lateral and dis- 

 tant, that of the posterior side being composed of two 

 or three distinct, unequal impressions ; the animal is 

 destitute of any projecting tube or siphon; the foot is 

 elongated, transverse, and rounded, which it protrudes 

 between the valves, and uses to change its position. 

 Anadonta cygnea, pi. 24, f. 43. a, the mouth, into which 

 a bristle has been introduced; b, the stomach, under 

 which the intestine makes five turns in the foot amidst 

 the ovary, and then as it returns, runs posteriorly along 

 the back of the animal beneath the hinge, and above 

 the respiratory organs, passing through the midst of the 

 heart, c, and opening at d above the posterior muscle 

 closing the shells, beneath the small tube of the cloak. 

 This description applies generally to most trivalves, but 

 in the oyster the rectum does not pass through the 

 heart, tfnio pictorum, pi. 24, f. 49. 



FAMILY II. TRIGONACEA. 



Primary teeth of the shell lamelliform, and striated 

 transversely; ligament exterior. Castalia ambiyua, pi. 

 32. f. 35. 



FAMILY III. ARCACEA. 



Primary teeth of the shell small, numerous, and dis- 

 posed in a line on each valve of the shell, either in a 

 straight or interrupted series. The valves do not close 

 perfectly in the centre, in consequence of having a horny 

 plate or'fillet placed before the abdomen of the animal, 

 that serves as a foot, and by which it adheres to sub- 

 merged bodies. Area Note, pi. 24, f. 59. Area larbata, 

 pi. 24, f. 70. a, the foot. Pectunculus Pilosus, pi. 24, f. 

 56. f . 74, animal of do. 



FAMILY IV. CARDIACEA. 



Primary teeth of the shell irregular, both in their 

 form and situation, and having generally one or two 

 lateral teeth. In the animal, the mantle is open before, 

 and there are besides, two separate apertures, one for 

 respiration and the other for the fteces, which are pro- 

 longed in tubes, sometimes distinct, and at others 

 united in one single mass. There is always a transverse 

 muscle at each extremity, and a foot generally used 

 for locomotion. Those which are provided with long 

 tubes always live in ooze or sand. Isocardia cor, pi. 24, 

 f. 98. Cardium rusticum, pi. 24, f. 60. a e, the tracheae ; 

 a, part of the abdomen; 1 6, the branchiae; c, the adduc- 

 tor muscle. 



FAMILY V. CONCHACEA. 



Shell with three primary teeth, at least in one valve, 

 and the other with a similar number, or less in some 

 species ; sometimes provided with lateral teeth. The 

 animal has generally two tubes or siphons projecting 

 from its mantle, one of which answers the purpose of a 

 passage for the water to the branchiae, and the other 

 for dejected matters; the foot is lamelliform. All the 

 animals of this family inhabit sand or mud. 



Subdivision I. Generally destitute of lateral teeth ; 

 inhabiting the ocean ; siphons of the animal elongated 

 and unequal; the foot broad and projecting. Venus 

 Verrucnsa, pi. 24, f. 39. CytJierea Chione, pi. 24, f. 48. 

 Artemis orbiculata, pi. 24, f. 53. 



Subdivision II. Fresh water shells covered with a 

 spurious epidermis, and having lateral teeth. Cyclas 

 cornea, pi. 24, f. 50. Piscidium obliauum, pi. 24. f. 51. 



SECTION III. TENUIPEDES. 

 FAMILY I. NYMPHACEA. 



Shell never having more than two primary teeth in 

 each valve; frequently gaping at its lateral extremities; 

 ligament exterior. The foot of the animal is small, and 

 often compressed ; the species all inhabit the shores of 

 the sea. Donax tmncuhts, pi. 24, f. 46. The animal has 

 two long slender tubes, which are received into a sinus 

 of the mantle; and provided with a broad foot. F. 47. 

 The right valve removed, a a, the adductor muscles; 

 A the foot; s ., the right lip; m n. the two lobes of the 

 branchiae ;b d,ce, the two tracheae; the left vein is at i. 

 Tellina pianola, pi. 24, f. 97. 



FAMILY II. LITHOPHAGI. 



The shells gape more or less at their anterior side ; 

 ligament exterior; they bore into rocks and clay, with- 

 out the aid of accessory shells, but by what means 

 naturalists have not yet discovered. Venerupis irus, pi. 

 24 f. 69. 



FAMILY III. CORBULACEA. 



Shell inequivalve, with the ligament placed interiorly. 

 / andora rostrala, pi. 32, f. 72. 



FAMILY IV. MACTRACEA. 



Shell equivalve, frequently gaping at the lateral ex- 

 tremities; ligament placed interiorly, or partly exter- 

 iorly; animal with the foot small and compressed; the 

 tubes are united and short. Mactra Neapolitana, pi. 24, 

 f. 40. b c, the trachaea; rf, the foot. 



