ACEPHALA TESTACEA. 401 



equal, and of a fibrous texture. They appear to have had a bys- 

 sus(l). 



PuLviNiTEs, Defr. 

 A regularly triangular shell, in which the few depressions diverge 

 from the summit on the inside. The impression is found in chalk(2). 



In the second subdivision of the Ostraceaj as well as in al- 

 most all the bivalves which follow, besides the single trans- 

 verse muscular mpss of the preceding genera^ there is a fasci- 

 culus which is placed before the mouth, and extends from one 

 valve to the other. It is apparently in this 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 Ostrese in having two muscular 

 impressions. The animal is not seen to produce a byssus(3). 

 They have lately been discovered in the Upper Nile(4), 



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 mus- 

 cle is excessively small. 



The species with less salient ears form the Pintadinve, Lam., 

 or Margarit.*:, Leach. 



The most celebrated, Mytilus margaritiferus, L., Chemn. 

 VIII, Ixxx, 717, 721, has nearly a semicircular shell, greenish 

 without, and ornamented with the most beautiful nacre within. 

 The latter is employed in the arts, and it is from the extravasa- 

 tion of this substance that are produced the oriental or fine 

 pearls, taken by the divers at Ceylon, in the Persian Gulf, 8cc. 

 The name of Avicula is appropriated to such as have more 



(1) Catillus Cuvieri, Brong-., Cuv., Oss. Foss., II, pi. iv, f. 10. 



(2) Pulvinites Mansonii, Defr., Ulainv., Malac, Ixii, bis, 3. 



(3) Etheria elliptica. Lam., Ann. du Mus. X, pi. xxix, and xxxi; — Etk. trigo- 

 nula, lb., pi. XXX; — Eth. seminularis, lb., pi. xxxii, f. 1, 2; — Eth. transversa, lb., 

 f. 3, 4. 



(4) Eth. Caillaudi, Voy. de Caillaud a Meroe, II, pi. Ixi, f. 2, 3. 



Vol. IL— 3 A 



