202 NOTES ON THE PERMIAN SYSTEM OF 



to the A. ant'iqua of the Durham magnesian limestone. A. inflata 

 is identical with the preceding. The hinge-area is very much 

 inclined, perhaps from the valves being partially open. The 

 only difference perceivable in A. discors is that the area is not 

 so much developed, and the umbones are closer in consequence. 

 The right valve also appears smaller than usual, but this may 

 be from compression. Two other specimens of A. antiqua in the 

 same collection were much larger and better preserved than the 

 preceding. They have the posterior margin also more forked, 

 and there is a deepish constriction running from the umbones 

 to the ventral margin." 



Great development of the hinge-area cannot be considered a 

 specific character in this species, nor in some of the following; 

 for when this area is greatly increased, there is generally a very 

 stunted growth of some other part of the shell. 



This species and the following may be very safely and most 

 conveniently placed in the above genus, " until more is known 

 of the general characters and chronogeny of a number of 

 palaeozoic fossils." There seems to be no necessity for instituting 

 a new one for their reception at present. 



In the compact-limestone rare, but rather plentiful in some 

 localities in the shell-limestone. In this district it has not yet 

 been detected in the higher members of the limestone, which 

 are probably the true equivalents of the Permian marls of Man- 

 chester and its neighbourhood, where this little shell seems to 

 be very common. 



2. G. CERATOrHAGA, Schlotll. 



It is of a more elegant form than the preceding, and the tumid 

 part of the shell is more distinctly characterised and produced 

 to a greater length posteriorly than in the G. antiqua. The 

 hinge-line is very long, and produced posteriorly to a fine point, 

 whence it curves beautifully downwards, forming a deej) curve 

 with the tumid part of the shell. The hinge-area is not so 

 broad and more parallel than in the foregoing species ; and 

 its cartilage- pits, placed directly across, and not obliquely, 

 as described in the " Monograph," have an oblong appear- 



