134 MONOMYARIA. 



Genus 30 HIPPOPUS — ZawarcA. 



Generic Character. — Equivalve, regular, inequilateral, 

 and transverse ; lunule closed; hinge with two compressed 

 unequal teeth; ligament marginal, and external. 



Distinguished from the Tridacna, by having the posterior slope 

 closed, or nearly so, and the inner margin dentated at that part. 

 The spines which arm the ribs are tubular, and are never arched or 

 vaulted. 



Hippopus maculatus The Spotted Hippopcs. Plate 



XV. tig. 18. Transversely ovate, ventricose, with scaly 

 ribs; and reddish purple spots; lunule heart-shaped, and 

 oblique; the margins very deeply crenulated. Eight inches 

 broad. Inhabits the Indian seas. 



Genus 31. — TRIDACNA Lamarck. 



Generic Character. — Shell regular, equivalve, inequilateral 

 and transverse ; a gaping space near the beaks ; hinge with 

 two compressed, unequal teeth ; ligament marginal and ex- 

 ternal. 



A bundle of tendinous fibres passes through the aperture of the pos- 

 terior slope, by which the animal affixes itself to rocks, where it re- 

 mains suspended. Although there is no nacreous substance in the 

 valves themselves, yet the animal sometimes produces very fine pearls, 

 and of large size. 



Tridacna gigas. — The Giant Tridacna. Plate XV. 

 fig. 21. Large, white, transversely ovate; with broad ribs, 

 provided with vaulted scales ; the posterior slope heart- 

 shaped and gaping. Two feet six inches long, and four feet 

 six inches broad; the largest of all known shells, sometimes 

 weighing above five hundred pounds. Inhabits the Indian 

 ocean. 



OEDER n.-DIMYAPJA. 



Shell having two separate lateral muscular impressions. 



