FAMILY 1. TR1DACNACEA. 133 



umbones is generally rather large, and has a wide opening for the passage 

 of a strong tendinous byssus, by which the animal fixes itself to the 

 rocks. The hinge has two blunt, unequal, compressed teeth in each 

 valve, the anterior in one valve being inserted between those of the 

 opposite valve. The interior of the shell is of an opake marble-white, 

 and exhibits the muscular impression spreading out towards the ventral 

 margin. The ligament is submarginal and external. 



One species, the Tridacna gigas, is by far the most gigantic of all mol- 

 lusks ; it is said to possess immense muscular power, with a shell so large 

 and ponderous as sometimes to exceed five hundred pounds in weight*. 



Example. 



PI. XCVII. Fig. 1. 



Tridacna elongata, Lamarck, Anim. sans vert., new edit., vol. vii. 

 p. 9. Enc. Me'th., pi. 235. f. 4. 



HIPPOPUS, Lamarck. 



Testa regularis, aequivalvis, incequilateralis, extus tuberculis numerosis 

 imbricata ; lunula fere clausa. Cardo dentibus duobus in utraque 

 valva, compressis, inaequalibus, anticis insertis. Impressio musculi 

 adhserentis subcentralis. Ligamentum externum, marginale. 



Although the Tridacna; and Hippopi were separated from the Chamee 

 by Klein under the title of Chanuctrachea, Linmeus continued to follow 

 the arrangement of the earlier naturalists. Bruguiere adopted the genus 

 as proposed by Klein, substituting the appellation of Tridacna ; but La- 

 marck, upon noting that the lunular opening for the passage of the foot 



* The Tridacna gigas appears to be die lleXwpi'as of the ancients : this title, which is used 

 by Nicander (vide title-page) and some of the early Greek poets, is derived either from the 

 word Tre\u>pios, gigas, or from Pelorus in Sicily, on the shores of the Mediterranean, where 

 the shell is found. 



