149 



Dimensions. — Lengtli, 5'5 ; width, 5 millimetres. 

 Localities. — jS"ot uncommon in the lowest beds, Blanche 

 Point, Aldinga, and Adelaide bore. 



G-ENTJS Mytilicaedia. 



SX>5"0PSIS OF SPECIES. 



Posterior side widened ; ribs 21. J\I. alata. 



Posterior side narrowed ; ribs narrow, 15. M. compta. 



Posterior side narrowed ; ribs broad, 12. M. platycostata. 



Shell trapeziform, uniformly ribbed. 21. curta. 



Mytilicardia alata, sine. nov. Plate ii., fig. 12. 



Shell ovately-oblong, somewhat depressed; anterior side 

 Tery short, truncated ; widely rounded and compressed pos- 

 teriorly; ventral margin nearly straight, inconspicuously 

 sinuated. 



Surface radially ribbed, with lamellar or spiny projecting 

 scales, and undulating concentric lines of growth. Pour ribs 

 of large dimensions occupy the ridge extending from the umbo 

 to the post-ventral angle; these are broad, rounded, not much 

 elevated, rather abruptly narrowed to form a surmounting rib, 

 which carries the distant spiny scales. The anterior ribs are 

 twelve in number, densely covered with scaly spines. The 

 posterior ridge is succeeded by a broad space with two flat 

 ribs, posterior to which are three compressed ribs with imbri- 

 cating spinous scales. Inner margin of valves coarsely crenu- 

 late. 



Dimensions. — Length, 25 ; greatest width, 20 ; width at 

 umbo, 12 millimetres. 



Locality. — Adelaide bore. 



This species is distinguished from j\I. crassicostata, and allied 

 species possessing lamellar scales on the ribs, especially 

 by its wing-like posterior slope ; in the longitudinal constric- 

 tion of the primary ribs it resembles M. Noixb LLollandice, 



Mytilicardia compta, spec. nov. Plate xii., fig. 2. 



Shell elongately ovate, somewhat depressed ; ventral margin 

 inconspicuously sinuated, in other respects similar to JH. crassi- 

 costata, Lamk. There are four primary subaugulated ribs, pos- 

 terior to which are two narrow ribs succeeded by two com- 

 pressed elevated ribs ; the anterior ribs are seven in number. 

 All the ribs are more or less raised into lamellar scales. 



Dimensions. — Length, 29 ; greatest width, 17 ; width at 

 umbo, 12 millimetres. 



Localities. — Muddy Creek ; oyster banks at jSTorth -west Bend 

 Head Station, on the Eiver Murray. 



