179 



MONOGEAPH OF THE GENERA 



TRIDACNA AND HIPPOPUS. 



Plates I., II., III., IV., V., VI. 



(PI. 485-489*.) 



Testa bivalvis, unimnsculosa, antice hians, ad marginera 

 hiatiis creiiata, plenimque ovata, costis latis plusminiisqtie 

 sqiiaraatis radiata ; squamis arcuatis, interdum magnis ; mar- 

 gine ventrali dentato-undatis. Cardo dente approximato imo in 

 utraqiie valva magno, remotis uno et duobus. Ligameutum 

 marginale. — The one large adductor muscle is to be found at 

 about two-thirds length from the posterior end, half way 

 between the dorsal and ventral margins. The open lunule is 

 for the passage of a strong byssus, by which the Tridacna 

 fixes its shell to the rocks. All tJie shells are ribbed, and 

 most of them have scales on the ribs, although when the 

 shells attain a large growth these scales are often eroded, 

 and in other cases their production is discontinued. 



Species. 



1. GiCfAS, f. 11, Linnceus. — Maxima ponderosa subovata 

 subaequilateralis, costis radiantibus septem elevatis loevibus vel 

 tenuissime laminatis ; margine ventrali acute et profunde \\n- 

 datim dentato, margine dorsali crassidentato, antice breviter 

 et rotunde liianti vix crenato. — Neither the shells referred to 

 by liamarck, nor that figiu-ed in Reeve's ' Icouica ' for 2\ 

 gigas (probably mutlca, Lamk.), will be found to agree with 

 the Linna^an description, nor is there any reason to believe that 

 either of them attains to gigantic dimensions. I have no 

 means of knowing whether the gigantic specimens weigliing 

 over 500 11)., of the ^Museum Ulricue, or tliose used as bciiltiers 

 at the Eglise de St. Sulpice in Paris, do or do not slmw signs 

 of having had scales ; but I agree with Mr. Ilanley in believing 



