72 



Adult shell cuneiform, somewhat elliptical, elongate, rather 

 thick, semitranslucent, of a dark-brown colour, greyish towards 

 the posterior margins where the lamellae are densely imbricat- 

 ing and exfoliating. 



In the anterior-third the valves are rather acutely convex, 

 becoming increasingly depressed posteriorly ; the axis of eleva- 

 tion is nearer the dorsal margin, and consequently the dorsal 

 slope is steeper and shorter than that on the ventral side, 

 which is undulatory ridged for about one-half or one-third of 

 the whole length from the apex ; posterior to the byssal orifice 

 the ridges are marginal. 



The dorsal margin nearly straight, varying from slightly 

 convex to slightly concave ; the ventral margin usually concave 

 about the byssal orifice, thence convex to about two-thirds its 

 length, where it commences to curve downwards to join the 

 posterior margin. The posterior margin arched, somewhat 

 produced, corresponding with the axis of elevation of the 

 valve. 



Surface ornamented with imbricating adpressed lamellae and 

 slight growth folds, especially on the anterior- ventral and-dorsal 

 slopes ; some individuals exhibit faint longitudinal ribs on the 

 dorsal slope. Interior bluish-brown. 



Dimensions of a large specimen. — Greatest length from the 

 apex, 280 millimetres (14| inches) ; dorso-ventral diameter at 

 right angles to the axis at two-thirds the whole length from the 

 apex, 153 millimetres (six inches) ; greatest sectional diameter 

 of both valves at about half the axial length, 51 millimetres 

 (two inches). 



Localities. — Vertically imbedded by their apices in the soft 

 bottom of sheltered bays, just below low-tide mark ; general 

 from Eucla to the South-East. 



This common so-called "razor-fish" has hitherto been known 

 as P. Zeylanica, which name was applied to it by Angas in 

 1865, on the supposition that it is identical with the N.Z. shell. 

 This identification is obviously erroneous, and is altogether in- 

 explicable, as the two species are so widely different. The true 

 P. Zeylanica has longitudinal ribs armed with close short semi- 

 cylindrical hollow spines, and has a truncated posterior margin. 



T. Woods (Proc. Tas. for 1875, p. 161) has evidently been 

 misled, for in his description of P. Tasmanica he says, that in 

 the few irregularly much-raised tubular scales it is quite dis- 

 tinct from P. Zeylanica, which is not uncommon in Australia. 

 This author would seem to have been unacquainted with that 

 species inasmuch as the characters he relies on by which to 

 separate it from the species herein named, P. inermis, are such 

 as ally it to P. Zeylanica, and raises a doubt if it be distinct 

 from it. 



