206 STUDIES ox AUSTRALIAN MOLLUSCA, VIII., 



which in old age are apt to be worn away. Epidermis dense 

 black, laminate, thinner and usually denuded on the anterior 

 exposure; in youth the epidermis appears as bristles. 



Interior porcelain-white, sometimes green under the umbo; the 

 larger ribs imprint within conical crenulations between the 

 pallial line and the posterior ventral margin; pallial sinus very 

 slight. Hinge-line straight and narrow, armed with about 

 twenty-one anterior and twenty-six posterior teeth. In the centre 

 is a sharp break of gauge, the posterior row being considerably 

 smaller, at each end the teeth increase in size and the outermost 

 tumble away from the vertical. Length 52; height 40; depth 

 of separate valve 17 mm. 



The species ranges from Bass Straits to Moreton Bay. 



Area liscJihei is gregarious and inhabits the soft mud-flats of 

 estuaries. It is sunk up to the umbo obliquely in the mud, and 

 often masked by a tuft of Ulva planted on the anterior end. 

 The swell amidships serves to support the organism from sinking 

 too deeply. Its habits are sedentary. 



The asymmetry and wedge-shape appear to have been induced 

 by environment. Parallel development has been carried to an 

 extreme in the case of Area tortnosa and A. semitorta, which 

 likewise live sunk nearly upright In the mud. Their sharp pos- 

 terior ends are planted deep in the mud, and the anterior 

 inflations serve to buoy them up on the surface. 



The growth of Area lischhei involves a change in contour, 

 advancing from the symmetry of infancy to asymmetry of youth, 

 and greater degrees of asymmetry in adolescence and senility. 

 A young shell, 3 mm. long, is drawn (fig. 33) to show how the 

 balance of growth is already upset by the preponderance of the 

 posterior side. In another example (fig. 32), 11 mm. long, pro- 

 gress in the same direction is continued. It is interesting to 

 observe that an impressed umbonal ray (perhaps an ancestral 

 feature) like that possessed by A. anfiquata, Linn., is a prominent 

 feature in the younger shell, is perceptible in the elder, and has 

 disappeared in the adult stage. 



