LlTHODOMUS.- 



-MOLLUSCA.- 



-PiNNAD.E. 



373 



wedge-shaped behimi, aud with slightly prominent 

 beaks. The valves are covered externally with a 

 dark, thick, horny epidermis, and are pearly within. 

 The species are boring shells, and are not numerons. 

 They are found principally in the West Indies .ind 

 New Zealand; but one well-known species is a native 

 of the Mediterranean, and has often been quoted by 

 geologists as proving the fiict of a change of the level 

 in sea-coasts in modern times. In the Bay of Naples, 

 near to Pozzuolo, is situate the ruined temple of Jupiter 

 Serapis. Only three pillars remain standing, and at the 

 height of ten feet above the base each pillar exhi- 

 bits a zone of six feet in height, where the marble has 

 been perforated by these shells. The holes are four 

 inches in depth ; and it is observed that the nodules 



of quartz and felspar, which sometimes occur in the 

 hard limestone of the pillars, are untouched. Cuvier 

 informs us that in their young state, the Lithodumi 

 suspend themselves to rooks by a byssus ; and that it 

 is not till they are adult that thej' begin to perforate. 

 The cavities which they form in limestone rocks, corals, 

 shells, &c., are shaped like the shell, and do not admit 

 of free rotatory motion. 



LITHODOMUS DACTYLTJS, the Mediterranean species, 

 is ealeti by the natives residing on the coasts. It is 

 collected in quantifies by the fishermen, and sold as a 

 luxury in the markets. It possesses a remarkable phos- 

 phorescent quality. " The fishermen place the animal 

 in the sun, and with it besmear their hands and faces at 

 night, so as to illuminate them as with phosphorus." 



ORDER III.— PECTINACEA. 



Family— TRIGONIID.!. 



The Trigonias have the shells thick, closed, and 

 trigonal or somewhat heart-shaped, with the beaks 

 directed posteriorly. The hinge is composed of two 

 diverging cardinal teeth in the right valve, transversely 

 deeply striated on both sides ; and four in the left, stri- 

 ated on one side only. Externally the valves are 

 ornamented with radiating tuberculated ribs, while in- 

 ternally the surface is beautifully iridescent and pearly. 



The species are few in number, and are all from the 

 Australian seas. The genus is represented in Plate 

 11, fig. 18, by Trigonia pectinata. 



Family— .\RCAD.E. 



The Arks have a large foot, which often secretes a 

 laminated byssus — thus adapting the animal to anchor 

 itself. The shell is angular, elongated, or roundish, 

 sometimes closed, and sometimes gaping inferiorly. 

 Tlie hinge consists of a number of transverse, inter- 

 linking teeth, placed in a straight line, which appear 

 to be formed by the subdivision of two elongate lateral 

 teeth. In general the cartilage is external, arising 

 from diverging angular lines marked upon the facet, 

 formed by the gradual thickening of the dorsal edges, 

 which causes the umbones to be separated from each 

 otlier as the shell enlarges. The byssus in this family, 

 instead of being composed of silky threads is in form 

 of a horny cone, composed of numerous thin plates, 

 occasionally becoming solid and calcareous, and which 

 the animal can cast off and form again with great 

 rapidity. 



THE TYPICAL ARKS (represented in Plate 11, fig. 

 19, by Area Noa) have the shell equivalve or nearly so, 

 tlie valves covered with an epidermis, and strongly 

 ribbed or cancellated externally ; the margins are 

 smooth or dentated, gajiing or closed, and sinuated 

 ventrally ; the hinge margiii is straight, and the carti- 

 lage is placed in small marginal pits. The species are 

 numerous, and attach themselves to rocks, stones, &c., 

 by means of their peculiar byssus, or conceal them- 

 selves in holes and crevices. The general form and 



the particular arrangement of the hinge teeth vary con- 

 siderably in the dillerent species, and various genera 

 have accordingly been formed to receive them. 



Genus Pectunculus. — This genus has an orbicu- 

 lar, nearly equilateral shell; the valves closed, smooth, 

 or radiateiy striated, and invested with a fimbriated 

 epidermis. The beaks are separated by a grooved, 

 lanceolate, ligamental area. The hinge is semicircular, 

 and the teeth are small and transverse. The species 

 number about fifty, one half of which are natives ot 

 America ; the others being found in Europe, India, and 

 New Zealand. They inhabit muddy ami sandy coasts, 

 ranging from eight to sixty, and, in some instances, to 

 a hundred and twenty fathoms. 



Gekus Nucula. — This genus has a trigonal or 

 obliquely ovate shell, with the beaks turned towards 

 the short posterior side. The valves are smooth or 

 sculptured, and covered externally with a smooth olive 

 epidermis, while the interior is pearly, and has the 

 margins creiuilated. The hinge has a prominent in- 

 ternal cartilage pit, and the teeth form a range of 

 comb-like denticles on each side. The species are 

 rather numerous. They do not spin a bj-ssus, but use 

 their foot for burrowing, and are able to crawl upon it 

 like a Gasteropod ; indeed Professor Forbes tells us 

 that he has seen one of our native species creep up the 

 side of a glass of sea-water. 



Family— PINNAD^. 



The species of this family have a large foot which 

 secretes a powerful and very fine silky byssus. The 

 shell is large, triangidar, and of a prismatic texture. 

 The valves are thin in general, more or less fragile, 

 and gaping posteriorly; externally they are smooth, 

 or scaly, or obliquely furrowed, and internally shining 

 and nacred. The beaks are terminal, the hinge straight, 

 long, and toothless, and the ligament linear and internal. 

 About thirty species of Pinna? have been described, and 

 they are found in nearly all seas, ranging from low 

 water to sixty fathoms. They usually live buried in 

 the sand, or between cracks in rocks, with their shaip- 

 edged. gaping, truncated ends just above the surface. 



