THE UN 10 FAMILY. 



241 



ANATOMY OF TKIGONIA PECTINATA. 1 



Cucullea resembles Byssoarca, but the valves are squarish and striated, and fit close together. 

 Two species are found living in Nicobar, China, &c. 



The characters of the genus Pectunculus are shell nearly circular, valves equal, striated 

 radially, hinge thick, with a row of teeth, and a ligament area between the beaks of the valves. 

 The animal has a large foot, and the mantle is open and provided with ocelli Fifty-eight species 

 are found living in the West Indies, Britain, New Zealand, tc. 



Genus Limopsis is like a small oblique Pectunculus, with a 

 triangular cartilage pit in the centre of its hinge. Four species are 

 known living in the Red Sea, Japan, Britain, &c. 



Genus Nucula. In this genus the valves are somewhat triangu- 

 lar, with their beaks turned backward; interior of valves pearly, 

 hinge with a large cartilage pit, and numerous sharp teeth on each 

 side. The Nuculse are burro wers, and have very wide distribution, 

 from Norway to Japan, living from five to more than 100 fathoms 

 in depth. Seventy living species are known. 



Genus Leda, resembles Nucula, but the shell is more elongated 



and pointed behind. It is found in the Northern and Arctic seas, living from ten to 200 fathoms. 



Genus Solenella. In this gen vis the shell is nearly oval ; the valves are pearly within ; the hinge 



ligament is external ; the line of the mantle has a large and deep fold ; the siphonal tubes are joined 



together ; they are long and slender, and can be drawn completely into 

 the shell. They are found living at Valparaiso, New Zealand, &c. 



Genus Solemya. The valves of the shell are somewhat cylindri- 

 cal and elongated, and gape at each end. They are covered with a 

 dark horny epidermis, which overlaps the margins. There are no 

 hinge teeth. Four species only are known in America, Africa, and 

 the Canaries. 



FAMILY VII. TRIGONLiUm 



The shells of this family have the valves equal, triangular in 

 form, closely fitting, with the umbones of the valves turned back- 

 wards. The hinge teeth are diverging, the border of the mantle is 

 simple, the interior of the shells pearly; the hinge ligament is 

 external. The foot is long and curved; there are two gills on 

 each side, and the mantle is open. 



In the genus Trigonia the shell is thick and ornamented with tubercles, or with ribs 

 arranged in concentric or radiating lines ; the posterior side of the valves is angular the 

 shells are almost entirely composed of pearl. Like the young Pectens, the 

 Trigonice are very active bivalves. A Trigonia, taken alive from the dredge 

 by Mr. S. Stutchbury, and placed on the gunwale of the boat, leapt over- 

 board, clearing a ledge of four inches. They are probably migratory, as 

 in dredging for them it is very uncertain where they may be obtained, though 

 they abound in some parts of Sydney Harbour. Trigonia is almost an 

 extinct form, three species or varieties only being known living in Australia, 

 whilst more than one hundred are found fossil, widely distributed over the 

 globe. 



FAMILY VIII. UNIOMID^E. 

 The animal bears a pearly shell, with the mantle lobes freely open except 



behind, where they are united to form the branchial and excretory siphonal orifices, which are simply 

 pouted. The foot is large and free. 



* Explanation of the lettering in this figure : a, anterior adductor muscle ; d, posterior adductor muscle ; h I, hinge 

 ligament ; t t', pits for the reception of teeth in the right valve ; /, foot ; r, excreting orifice ; m, free margin of the mantle ; 

 o, the mouth ; p, line (corresponding with the pallial impression in the shell) from which the muscular fibres of the mantle 

 originate. The central portion of the mantle is thin and transparent. Through it are seen b r, the right branchial leaves ; 

 and 1 1, the labial tentacles of the right side of the mouth. The arrows indicate the points at which the respiratory currents 

 enter and escape. t /' /, hinge-teeth and sockets ; a' a, adductor muscles. 



221 



TKIGOX1A COSTATA. 



THIGONIA PECTIN ATA f 



