ACEPHALA TESTACEA. 275 



elongated in a transverse direction, represent the handle. It Inhabits the 

 Archipelago of India. 



Vulsella, Lam. 



A little salient plate inside of the hinge of each side, from one of which to 

 the other extends tlie ligament, otherwise similar to that of the Ostrese. 

 By the side of this plate is a notch for the byssus, as in the Mallei. The 

 shell is elongated in a direction perpendicular to tlie hinge. 

 The most known species inhabit the Indian Ocean. 



Perna, Brug. 

 Several parallel cavities across the hinge, opposed to each other in the two 

 valves, and lodging as many elastic ligaments; the irregular and foliaceous 

 shell marked on the anterior side and under the liinge by a notch traversed 

 by the byssus. The Pernae were also left by Linnsus among the Osti-ex. 

 This genus is now subdivided. 



In the second subdivision of the Ostracea, as vi^ell as in almost all 

 the bivalves which follow, besides the single transverse muscular 

 mass of the preceding genera, there is a fasciculus which is placed 

 before the mouth, and extends from one valve to the other. It is 

 apparently in tiiis subdivision that we must place the 



Etheria, Lam. 

 Large inequivalve shells, as irregular as those of the Ostrese, and more so-; 

 no teeth to the hinge; the ligament partly external and partly internal. 

 They differ from the Ostreae in having two muscular impressions. The ani- 

 mal is not seen to produce a byssus. 



They have lately been discovered in the Upper Nile. 



AvicuLA, Brug. 

 An equivalve shell with a rectilinear hinge, frequently extended into wings 

 by its extremities, furnished with a narrow and elongated ligament, and 

 sometimes with small notches near the mouth of the animal; in the anterior 

 side, a little beneath the angle of the side of the mouth, is a notch for the 

 byssus. The anterior transverse muscle is excessively small. 



Mytilus margaritiferus, L., has nearly a semicircular shell, greenish with- 

 out, and ornamented with the most beautiful nacre within. The latter is 

 employed in the arts, and it is from the extravasation of this substance that 

 are produced the oriental or fine pearls, taken by the divers at Ceylon, in 

 the Persian Gulf, &c. 



Pinna, Lin. 



The Pinnae have two equal valves, forming a segment of a cu-cle, or ref5em- 



