OSTKEA. 225 



2. OsTEEA. Foliaceous, irregular, hinge on a small triangular 



disc. Including Dendostrea, Ostraea^ Exogyra, G-ryplisea. 

 Fig. 180 to 183. 



3. Plactjna^ Two diverging ribs near the umbones. Fig. 



184. 



4. PLACiiNATroMiA. The same, but attached by fibres passing 



through a hole in one valre. Fig. 189 to 191, 



5. Anomia. 'No hinge ribs, attached by a bony substance 



passing through a hole in one valve. Fig. 186 to 188. 



6. YuL SELLA. Tongue- shaped, a ligamentary pit on the 



hinge. Fig. 185. 



7. Mflleria. Doubtful Fig. 192. 



OSTEACEA. Bl. The first family of the order Lamellibran- 

 chiata, Bl. containing the genera Anomia, Placuna, Harpax, 

 Ostrea (including Dendostrea, Sw.) and Gryphaea, 



OSTE/EA. Auct. {oarpeoy, ostreon, a bone.) Fam. Ostraeea 

 Lam. and BL — Bescr. Irregular^ inequivalve, generally inequi- 

 lateral, foiiaceous, attached by part of the lower valve ; hinge 

 sometimes slightly crenated; destitute of teeth; with a liga- 

 ment spread upon the lower part of a central, triangular area, 

 which is divided into three parts ; upper valve much flatter than 

 the lower ; muscular impressions one in each valve, large, sub- 

 central, sub-orbicular, with one very minute. — Ohs. The Liu- 

 nean genus Ostrea includes the Pectens and many other genera 

 so different from each other, that without any desire to increase 

 the number of genera, it was found necessary by subsequent 

 authors to separate them. The common Oyster is the type of 

 this genus as at present constituted, and is well known to be 

 abundant in various parts of the world. Those which depart 

 farthest from this type are the Gryphaea, Lam. with a pro- 

 minent, incurved umbo in the lower valve. The Dendostrea, 



. Sw. with margins characterized by strongly angulated folds, 

 throw out arms from the lower valve, by which they are attached 

 to stems of sea-weed, &c. Fig. 180, O. edulis. Fig. 181. O. 

 folium. (Dendostrea, Sw.) Fig. 182, Gryphaea incurva. Fig, 

 183, Exogyra conica. PI. x. 



