16 



original form figured and described by J. Sowerbj. (See 

 Mitchell's Three JExped. Aust.) and Strezlecki's Phys. Descrip. of 

 N. S. Wales and Tasmania, p. 272, plate x., figs. 2 and 3. Both 

 species are common in the limestones at Maria Island. P. 

 Konincki is much more compressed, with dorsal margin more 

 rounded or arched ; the sinus under umbo is more concave, 

 and relatively the thickness is not half so great as in P. 

 gloiosus. 



PACIITBOMrjs IIOBAETENSIS. (SOV. Sp.) 



Shell large, orbicular, thick, moderately convex, equilateral ; 

 beaks recurved, approximating, gibbose ; there is a prominent 

 saddle-shaped tooth immediately under beak of left valve, the 

 anterior wing- of which is prolonged, and runs some distance 

 parallel with curve of interior sinus ; anterior slope slightly 

 concave under beak ; posterior and dorsal margins almost 

 perfectly rounded ; surface of valves ornamented with 

 concentric stria. 



Length, 4|- inches ; breadth the same. Common 

 limestones of Upper Pal. age, at the Cascades, Hobart. 

 Pachydomus beds, Maria Island. 



The almost perfectly orbicular form of this 

 distinguishes it from all other allies. 



coarse 



in the 

 and in 



species 



Pamilv AsTAUTiDiE. Genus Pleueophoiius. (King.) 



Shell oblong ; dorsal area defined by a line or keel ; umbones 

 anterior depressed ; hinge teeth, 2-2; laterals, IT ; elongated 

 posterior ; anterior adductor depression deep, with a small 

 pedal scar close to it, and bounded posteriorly by a stron<* rib 

 from the hinge ; pallial line simple. 



PlETJEOPHOEUS QUINQUEOOSTATUS. (NOV. Sp.) 



Shell oblong-oval, compressed, very inequilateral ; surface 

 ornamented with fine concentric strias ; there are fine, some- 

 what broadly obsolete or flatfish ridges radiating diagonally 

 from umbo to margin across the posterior half of shell ; at the 

 junction of each ray the concentric strife are sharply angled. 



Length, 64 millimetres ; height, 27 mil. 



This is a very singular form, and easily distinguished 

 from P. Morrisi (de Koninck) by the compressed form and 

 the fine, flattened diagonal rays. Bridgewater limestone. (Up. 

 Palaeozoic.) 



(Hall.) (Cteno- 



Pamily Aecid2e. Genus Tellts-omta. 

 bonta, Salter.) 



Shell elongately oval, sub-equilateral, smooth, or finely 

 concentrically striate ; valves moderately convex ; hinge repre- 

 sented by two diverging comb-like denticulated margins without 



