OSTREA. 211 



1. Pedum. Flat, turned up at the sides, an hiatus for the 



passage of a byssus. A triangular disc on the hinge. Fig. 

 179. 



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



disc. Including Dendostrea, Ostraea, Exogyra, Gryphsea. 

 Fig. 180 to 183. 



3. Placuna. Two diverging ribs near the umbones. Fig. 



184. 



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



through a hole in one valve. Fig. 189 to 191. 



5. Anomia. No eostse, attached by a bony substance passing 



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



6. Vulsella. Tongue-shaped, a ligamentary pit on the hinge. 



Fig. 185. 



7. Mulleria. Doubtful. Fig. 192. 



OSTRACEA. BL The first family of the order Lamellibranchiata, 

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

 (including Dendostrea, Sw.) Gryphsea. To these may be added 

 Placunanomia, Brod. and Mulleria. 



OSTREA. Auct. (oarptov, ostreon, a bone.) Fam. Ostracea, Lam. 

 and Bl. — Descr. Irregular, inequi valve, generally inequilateral, 

 foliaceous, attached by part of the lower valve ; hinge sometimes 

 slightly crenated ; destitute of teeth ; with the ligament 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. — Obs. The Linnsean 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 furthest from 

 this type are the Gryphsea, Lam. with a prominent, incurved 

 umbo in the lower valve. The Dendostrea, Sw. with margins 



p 2 



