532 ARCIDiR. 



Syn. Hippagus, Phil., not Lea. Trigonulina, D'Orb. 



Ex. V. ornata, D'Orbigny, pi. 124, fig. 2, 2, a. 



The only recent species of this genus at present known 

 was first met with by D'Orbigny in South America, and 

 described by him under the name of Trigonulina ornata. 

 The same species was subsequently named by Adams and 

 Reeve in the " Zool. of the Samar." H. novemcostatus, from 

 a specimen dredged from deep water in the China seas. 



Fam. AECIDiE. 



Animal oblong. Mantle freely open, simple or fringed ; 

 no siphons. Mouth surrounded by labia formed out of the 

 extremities of the branchiae; no true palps. Foot large, 

 oblong, bent, grooved throughout its length so as to form a 

 disk with plain or slightly-crimped margins. 



Shell not pearly within, closed or gaping inferiorly, with 

 two adductor muscles. Hinge with two teeth, each divided 

 into transverse, interlocking plates ; hinge-margin straight, 

 curved, or angular. 



Dr. Gray observes that, in the more typical form of this 

 family, the hinge consists of two oblong or linear teeth 

 in each valve, one placed on each side of the line directly 

 under the umbo of the shell. These teeth are divided 

 transversely into cross ridges, alternating and interlocking 

 with cross ridges of the teeth of the opposite valve, and 

 may be compared with the lateral teeth of Sjrisula in Mac- 

 tridce and Meretrix in Veneridw, but more especially with 

 the teeth of the genus Trigonia ; only here, instead of the 

 teeth being grooved on the sides, the grooves are suffi- 

 ciently deep to divide them in the manner described. 



