306 Mr. R. Howse on the Permian System of the 



in number, are, from increasing in width with the growth of the 

 hinge-margin, rather triangular, and are placed directly across 

 the area. They are never oblique in the specimens I have 

 examined, as represented in the * Perm. Mon.,' and the first pit 

 invariably stretches directly between the umbones. In the 

 clumsily-grown tumida, which is only a stunted form of this 

 species, they are placed closer together and are more numerous, 

 though the hinge-line is much shorter. In the Table, the 

 Bakevellia carinata, King, has been placed erroneously as a 

 synonym of the next shell ; but it is only the young of the pre- 

 sent species, which has oftentimes two strong, raised, diverging 

 lines running from the umbones to each side of the byssal 

 notch. 



Through the kindness of Mr. Binney I have made the follow- 

 ing notes on Capt. Brown^s species of Avicula : — 



The three specimens of Avicula Binneyi belong undoubtedly 

 to the A, antiqua 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 ^. 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. There seems to be no 

 necessity for instituting a new one for their reception. 



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. 



20. Gervillia ceratophaga, Schloth. — It is of a more ele- 

 gant form than the preceding, and the tumid part of the shell is 

 more distinctly characterized and produced to a greater length 

 posteriorly than in the G. antiqua. The hinge-line is very long, 

 and produced posteriorly to a line point, whence it curves beau- 

 tifully downwards, forming a deep curve with the tumid part of the 



