of the Rocks associated with the Coal of Australia. 303 



the genus generally, but the dental impressions are obscure. 

 Length 1 inch 1 line, width 2 inches 2 lines. 

 From the sandstone of Wollongong, N. S. Wales. 



Notomya (M'Coy), new genus. (Etym. voto?, auster, and My a.) 



Gen. Char. Shell transversely ovate, equivalve, inequilateral, 

 compressed, greatest thickness behind the middle of the shell ; 

 gaping slightly at both ends ; beaks small, compressed ; car- 

 dinal slope not distinguished from the sides of the shell ; shell 

 thick, surface concentrically lineated ; ligament external, large. 

 Cast : a wide shallow furrow runs obliquely from the beak about 

 half-way towards the ventral margin ; a shallow spoon-shaped 

 hollow extends from the beak to the impression of the poste- 

 rior adductor muscle, bounded by a low ridge on each side in 

 each valve ; traces of a simple cardinal tooth beneath the beak of 

 the right valve ; muscular impressions deep ; anterior adductor 

 large ovate, not attenuated above ; posterior adductor broadly 

 lunate ; retractor of the foot small, oval, immediately over the 

 anterior adductor; pallial impression with a small rounded 

 sinus before joining the posterior adductor. 



It is with those Muschelkalk Myacites of Schlotheim and 

 Bronn, and such like forms, of which M. Agassiz, in his f Etudes 

 Critiques sur les Mollusques Fossiles/ has composed his genus 

 Pleuromya, and with those forming his genus Gresslya, that 

 the present fossils have the strongest affinity. They are how- 

 ever perfectly distinct from those essentially Jurassic and Tri- 

 assic types, by the small size of the sinus in the pallial impres- 

 sion. In minor characters it differs from the Gresslyas in the 

 small size of the beaks, and the more compressed form of the sides 

 (the greatest thickness in Gresslya being always before the beaks, 

 and gradually diminishing towards the posterior end, while the 

 greatest thickness in Notomya is behind the beaks, depriving them 

 of the characteristic wedge-like form of Gresslya). The present 

 genus is destitute of the cardinal ridge in the right valve, so re- 

 markable in Gresslya, having in its place a shallow, attenuated, 

 ovate hollow, bounded by two obscure ridges in each valve, thus 

 approaching Pleuromya. The shell also is much thicker than in 

 the above genera, and the impressions of the muscular and pallial 

 scars much deeper and more strongly marked in consequence ; the 

 impression of the anterior adductor is pear-shaped, pointed and 

 attenuated above in Gresslya, but simply oval in Notomya. The 

 Pleuromya differ in nearly all the same points as Gresslya from 

 the present genus (except the cardinal ridge), and differ besides 

 in the elevation or upward curvature of the cardinal line and the 

 convexity of the posterior two-thirds of the ventral margin cor- 



