65 PAL/EONTOLOGY 



adductor from which a pallial line extends indistinctly 

 forward and ends in a very small anterior adductor 

 impression. The edge of the left valve overlaps that of 

 the right, and the small portion of the interior thus exposed 

 is seen to be nacreous. 



The genus Pseudomonotis is found principally in the 

 Triassic and Jurassic rocks, but allied genera range 

 from the Ordovician to the Recent period. Among its 

 Recent allies is the pearl " oyster " of the Pacific Ocean, 

 Meleagrina margaritifera, pearls being pathological secre- 

 tions of nacre around parasites or other irritating bodies, 

 and mother-of-pearl being the normal nacreous layer of 

 the shell. 



8. The genus Ostrea, the true oyster, ranges from 

 perhaps the Carboniferous period to the Recent. 0. 

 ventilabrum of the Oligocene (Fig. 20) is a convenient 

 species to describe, but (except for shape and ornament) 

 many other species will answer to the description. The 

 oysters are fixed (sessile) forms, and consequently very 

 inequivalve. The left valve is fixed by cementation to some 

 solid object often some other shell and becomes thick 

 and convex ; the right valve is nearly flat and forms a lid 

 to the left. The outline (of O. ventilabrum) is nearly semi- 

 circular, the posterior margin from the umbo back being 

 nearly straight. The height (in the morphological sense) 

 is greater than the length (length 6 cm., height 7 cm., 

 thickness 3 cm.) The shell is opisthogyral. In the left 

 valve, a variable area of the umbonal region is adherent 

 to some foreign body and shows only the impress of that 

 instead of its own ornamentation. Beyond this area the 



