13 



between hinge line and extremity of posterior ridge, rounded 

 inferiority ; ventral margin curved ; surface with distant, 

 obsolete, concentric lines of growth, which follow the curve of 

 the sinus above posterior ridge ; there are line microscopic 

 elongate punctures arranged quincuncialty as seen under the 

 lens. 



Greatest breadth at hinge line, 35 millimetres ; length, 32 

 mil. Upper Palaeozoic Marine beds, Porter's Hill, Hobart. 



Family Mytilid.?e. Genus Litiiodomus. (Cuvier.) 

 Shell cylindrical, inflated in front, wedge-shaped behind ; 

 epidermis thick, dark ; interior nacreous. (Tryon.) 



(irov. sp.) 

 inflated in front, con- 

 about one-third of the 



LlTHOBOMTTS AUSTBALIS. 



Shell very elongate, wedge-shaped, 

 traded anteriorly ; greatest breadth 



length 



, there is a shallow, lateral, diagonal depression pro- 

 ceeding from appoint near umbo to margin, forming a broad, 

 shallow concavity in the latter ; surface of valves marked with 

 distinct concentric lines of growth which coalesce anteriorly. 



Length, 77 millimetres; breadth, 28 mil. 



One slightly imperfect specimen in Tasmanian Museum, 

 from Bridgewater limestone. (XJp. Pal.) 



Family Mytilldje. Genus Mobiomoepiia, (Hall.) 

 Shell equivalve, very inequilateral, compressed sub- 

 ovate ; beaks small, compressed within the anterior third 

 of the length ; surface with rugose or undulating concentric 

 Striae, usually coalescing or fasiculating anteriorly; valve 

 crossed, crossed obliquely by a more or less distinct sinus 

 constructing the basal margin ; anterior end rounded ; 

 hinge with a single, strong, wedge-shaped tooth in the left 

 valve, and a corresponding cavity in the right valve ; no 

 lateral teeth ; ligament external, attached to the thickened 

 margin of the shell ; pallial line entire. (Tryon.) 



MODIOMOBPHA OKNATISSIMA. (NOT. Sp.) 



Shell very inequilateral, compressed, mytiliform, subovate, 

 nearly twice as long as broad ; beaks minute, situated near the 

 crown_ of the anterior side, which is comparatively narrow ; 

 Posterior side broadly rounded ; dorsal margin gently arched 

 towards posterior; ventral margin slightly concave near the 

 middle ; surface ornamented with somewhat distant rugose or 

 Undulating lines of growth, which coalesce towards margins 

 anteriorly ; interspaces marked with microscopic concentric 

 ^trias, _ and these again are in manv casts crossed with fine 

 radiating lines forming a fine network. 



Greatest length, 42 millimetres ; greatest breadth, 20 mil. 



Common as casts in Upper Marine beds (Up. Pal.), 

 Sorter's Hill, Hobart. 



