140 ADDITIONS TO THE LIST OF TASMANIAJST FOSSILS. . 



mesial fold of the dorsal valve is more often composed of a 

 single rib which is much larger than those situated on. the lateral 

 portions of the shell; its crest being in general rounded 

 from the umbone to about half its length, when it gradually 

 becomes more and more flattened as it approaches the frontal 

 margin, but at times it remains angular during its entire 

 length, with a tendency to the formation of a rudimentary 

 plait on either of its slopes, so that in these rarer cases the 

 fold assumes towards the front an obscurely triplicated 

 appearance. The sinus in the ventral valve is deep, acute, 

 and generally simple, but also more rarely interrupted by a 

 rudimentary rib, which becomes visible in the proximity 

 of the front. The valves are ornamented by from S to 12 

 angular ribs, which are, as well as the sinus and fold, inter- 

 cepted by closely disposed, concentric, scale-like laminae. The 

 surface of the shell is also closely beset by numerous small 

 granular (spinose) asperities ; the shell-structure being likewise 

 perforated by minute tubili or perforations. 



In the interior of the ventral valve there exists a sharp 

 •elevated mesial septum, which rises from the bottom of the 

 valve, and partly divides the spiral cones. Dimensions very 

 variable. Three examples, of which the first two are Sowerby's 

 original types, have afforded the following measurements : — 

 Length, 9 ; width, 13 ; depth, 8 lines. 

 » 6 „ 11 „ 6 „ 

 jj 5 „ o „ o ,, 



Spirifera duplicicosta, Phillips. 



Transversely sub-rhomb oidal when adult, longer than wide, 

 or almost circular when quite young ; valves moderately convex, 

 with a more or less produced mesial fold in the dorsal, and a 

 corresponding sinus in the ventral one. The hinge-line is 

 shorter than the width of the shell, the area of moderate 

 breadth, beak incurved. Valves ornamented by numerous 

 radiating ribs, which rapidly augment at various distances from 

 the beaks by intercalation as well as bifurcation. Two examples 

 have afforded the following measurements ; — 



Length, 16 ; width, 20 ; depth, 11 lines. 

 iRi I7i loi 



Spirifera alata, ScTilotheim. 



S. alata varies considerably in shape, according to age and 

 individual. When adult or full grown it is transversely fusi- 

 form, being twice and even three times as wide as long (PI. 1, figs. 

 23 and 27). Yalves convex, deepest at a short distance from 

 the umbone ; hinge-line as long as the greatest width of the 

 shell, the cardinal extremities being more or less attenuated in 

 •different individuals. The area is wide with sub-parallel sides ; 

 fissure triangular, and in great measure covered by a convex 

 pseudo-deltidium ; a narrow rudimentary area may be seen 



