170 
Shape. — This varies a good deal, as is uoted in Conch. 
Cab., Band x., Abtheil i., page 2 ; Taf. i., fig. 1 ; Taf. vi., 
fig. 1, 1886, where two figures are given, one of a shell 88 mm. 
by 75 by 42, and another much produced posteriorly, 98 by 
73 by 50. One from Port Lincoln, a rounded form, is 112 mm. 
by 93, while another very produced behind is 115 mm. by 90. 
This is not merely a senile tendency, for the difference in con- 
tour is found in young shells, and also in those of equal size 
and api^arently of similar age. In the produced individuals 
the ventral outline, instead of being uniformly convex as far 
as the postero-inferior angle, may be somewhat concave in 
front of this. 
Thickness.- It is very solid; the heaviest shell we have 
on our coast; it may weigh 10'75 ounces. Often growth in 
superficial area ceases after a time, and then the thickness 
greatly increases. Thus a shell only 37 inches long and 3*1 
deep is 2 05 in section, and weighs 10' 75 ounces, whereas 
another 4*25 inches long and 3'5 deep is only 1*9 inches in sec- 
tion, and weighs but 8 ounces. The volume of the contained 
mollusc actually diminishes, the thickening taking place at 
its expense. The muscular impressions appear deeply exca- 
vated then, owing to the heaping up of shelly material around 
the adductor muscles beneath the mantle. The ventral mar- 
gin, instead of being sharp, is flattened for as much as an 
inch, nearly at right angles to the external surface, and is in 
some cases even incurved. 
Periostracum.—H\\\^ is very durable, and even in dead 
and decaying valves is frequently present, and allows very 
fair cabinet specimens to be prepared from very unpromising 
material by a little careful scraping. It disappears sometimes 
first at the umbos and the subjacent surface, then erodes 
deeply. My largest individual, taken alive, has only a little 
of its epidermis remaining along the ventral and posterior 
borders, and its face value has been thus greatly depreciated. 
Interior. — This is smooth down to the pallial line, which 
is slightly crinkled, and thence on there are radial striae 
which fade out towards the ventral margin. The older the 
shell the deeper is the pallial line, and wider posteriorly, and 
more markedly crinkled, and the more rugose become the 
radial striae beyond. 
Colour. — The interior is white, with a beautiful glisten- 
ing chestnut or burnt-umber colouring of certain parts. The 
frequency and depth of tinting of these parts is in the fol- 
lowing order : — The posterior adductor scar, the posterior part 
of the pallial line, the anterior portion of the anterior adduc- 
tor scar, the posterior margin, the ventral margin, and the 
posterior part of the cartilage pit and hinge plate. Some- 
