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The nature of the substance placed under this vague 

 designation and description is entirely unknown. It bears 

 no resemblance in its structure to any known testaceous, 

 crustaceous, or bony fabric. Its appearance, as fixed in 

 the calcareous fissile stone of Pappenheim, in the above 

 figure, gives the idea of the valves of a shell ; but exami- 

 nation shows, that, if these valves were brought together, 

 they would touch only at two points, leaving all else, except 

 at the inferior line, open. 



The following shell is, in fact, neither univalve nor bi- 

 valve ; for, though a bivalve in form, the two parts are con- 

 nected, not by a hinge, but rather by suture. 



Hyalcea. An inequivalved, tumid, and transparent shell ; 

 tricuspidated at the base ; gaping beneath the prominent 

 beak. Recent. PL vii. fig. 7. 



We are indebted to the scientific and successful inquiries 

 of Mr. Sowerby, for the formation of the following genera, 

 in addition to cirrus, plagiostoma, and euomphalus, which 

 have been already noticed : 



Dianchora. An attached, inequivalved bivalve ; the at- 

 tached valve having an opening. instead of a beak, the other 

 beaked and eared; hinge without teeth. PL vii. fig. 11. 



Productus. An equilateral, unequal valved bivalve, with 

 a reflexed, more or less cylindrical margin ; hinge trans- 

 verse, linear ; beak imperforate ; one valve convex, the other 

 flat or concave, externally. Fossil. PL vii. fig. 13. 



This genus was formed, by Mr. Sowerby, for the re- 

 ception of those fossils which had been placed, by Mr. 

 Martin, under conchyliolithus (anomites) scabicrulus, Petrif. 

 Derb. and, of others, which he had obtained from the lime- 

 stone which forms a part of the independent coal-formation 

 of the Lothians, Scotland. 



Dd. 



