THE LAMELLIBRANCHIA 



65 



recognizable, but the right anterior ear, though small, is 

 quite distinct and has a well-marked notch below it. 

 Such a notch, as we know from recent forms, served for 

 the passage from the interior of the shell of the byssus (a 

 bunch of silky threads of conchiolin, secreted by the foot) 

 by which the mollusc could attach itself, temporarily or 

 permanently. The inequivalve character is here, as in 

 other lamellibranchs, associated with a fixed habit 

 of life. 



Between the umbo and the long hinge-line there is in 



FlG. 19. PSEUDOMONOTIS ECHINATA (J. DE C. SOWERBY), 



BATHONIAN. ( x 2.) 



a, Posterior view ; b, right side view: the black portion is the interior of 

 the left valve where it overlaps the right. At the upper end of this is 

 seen the very small right anterior ear, with the byssal notch below it ; 

 between it and the elevated umbo of the left valve is the amphidetic 

 area of the left valve. (Original.) 



each valve a long and low cardinal area, not separated into 

 lunule and escutcheon. As in Pectunculus, this is occupied 

 by an amphidetic ligament. 



It is not easy to clear the interior of specimens of 

 P. echinata, but from allied forms we may infer that hinge- 

 teeth are feebly developed or obsolete, and that the shell 

 is aniso- or heUromyarian, there being a large posterior 



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