SPIRIFER. 



TESTA transversa, sequilateralis, inaequivalvis, 

 cardine lineari, recto, ad iitrumque umbonum 

 latus extenso; umbonibus plus minusve distan- 

 tibus, area plan ulata intermedia, (aperturd tri- 

 gond centralis hyssum transmittentej appendi- 

 eulis duabus internis, spiraliter eonvolutis. 



Another Genus of Bivalve shells known only in a fossil 

 state which is peculiar to the oldest beds, and principally 

 characteristic of the Mountain or Carboniferous Limestone, 

 the Transition Limestone and the old red Sandstone ; we 

 believe not occurring in any bed above the Magnesian 

 Limestone. It appears to be nearly related to Terebratula, 

 from which, however, it is easily distinguished^ externally 

 by the area between the umbones, and internally by its 

 spiral ap]»endages. These appendages, which are com- 

 monly called cartilages are attached to and form a part 

 of the smaller valve, they are not, however, cartilagi- 

 nous, but consist of the same substance as the shelly in 

 Terebratula they are never spiral. 



Shell transverse, equilateral, inequivalve, with a 

 linear straight hinge, extending equally on both sides of 

 the umbones, which are more or less distant, being sepa- 

 rated by an intermediate, flat area; which varies much 

 in breadth according to the species. This area consists 

 of three triangular parts, two of which are lateral, (one 

 on each side) and the third which is central appears to 



