74 



16. gracilis, Phil. 



17. Gruneri, id. 



18. Hanleyanus, Bunk. 



19. laniger, id. 



20. lithophagus, Linn. 



21. Malaccanus, Reeve. 



22. Malayanus, Phil. 



23. nasutus 3 erf. 



24. obesus, id. 



25. pessulatus, Reeve. 



26. plumula, ZTawZ. 



27. rugiferus, Bunk. 



28. semigranatus, Reeve. 



29. splendidus, Bunk. 



30. stramineus, «?. 



31. subula, Reeve. 



32. teres, PM. 



33. truncata, Gray. 



Figure. 



LlTHODOMUS LITHOPHAGUS. PI. 31. Fig. 181. 



Sea Date. 



Shell of the Mediterranean 



Family 5. TRIDACNACEA. 



Shell ; either free or affixed, regular, equivalve, inequilateral, with 

 the lunule sometimes largely gaping ; hinge composed of two 

 compressed unequal teeth in each valve, of which the anterior 

 interlock ; muscular impression bipartite. 



The well-known Horse-foot and Giant Clams which constitute this fa- 

 mily are purely tropical mollusks. They have no representative on our 

 own shores, nor even in the Mediterranean, but dwell imbedded among the 

 coral reefs of the Indian and Pacific seas. From the observations of MM. 

 Quoy and Gaimard, whose interesting figure of the living Tridacna squa- 

 mosa we give at Plate N, it would appear that the animal's position in the 

 shell is the reverse of that in other bivalves, the foot of the Tridacna 

 being next the lunule of the shell, instead of in front. The hinge, composed 

 of two compressed unequal teeth in each valve, of which the front ones 

 interlock, is the same in both the genera, Tridacna and Hippopus ; but in 

 Hippopus the lunule does not gape, neither is there a byssus. 



Tridacna. Hippopds. 



Genus 1. TRIDACNA, Bruguiere. 



Animal ; oval, heart-shaped, having the lobes of the mantle united 

 almost throughout, three openings, two posterior and inferior for 

 evacuation and respiration, one anterior corresponding to the 

 opening of the lunule, giving passage to a stout cylindrical 

 byssus- spinning foot. 



Shell ; ovate, or oblong-ovate, gaping at the lunule, disposed in 



