Mr. J. E. Gray on the Arrangement of the Brachiopoda. 437 



Fam. 1. Thecid^ad.e. 



The animals are described by Philippi and D^Orbigny. The 

 genus Argiope, Be Longchamps = Megatheris, D^Orb. {Tereb. de- 

 trancata) is attached by a tendon passing out of a very large per- 

 foration below the beak of the dorsal valves ; Philippi confounds 

 this genus with his 0?'this, which is different from the Or this of 

 Dalman. Thecidcea has the shell attached by the truncated apex 

 of the dorsal valve, or it is free when the apex is produced and 

 entire. De Longchamps, who established the genus Argiope in 

 1839, pointed out the affinity of this genus to Thecidcea. 



Subclass 2. Helictopoda. 

 The oral arms are elongate, regularly spirally twisted when in 

 repose. The mantle lobes are merely applied to the inner sur- 

 face of the shell, and the substance of the valves is not pierced 

 with minute perforations, though the surface is sometimes spi- 

 nulose, the spines being only formed on the edge of the shell 

 while it is being increased in size. 



Order III. Sclerobrachia. 



The oral arms support a shelly band arising from the hinder 

 or cardinal edge of the ventral valve. 



Fam. 1. SPIRIFERIDiE. 



The oral arms very largely developed and supported the whole 

 of their length by a thin shelly? or cartilaginous? spirally 

 twisted plate. 



These shells are only known in the fossil state, but the spiral 

 supports of the arms are generally preserved, and may be disco- 

 vered by sections of the fossil, and are often to be seen in the 

 fractured specimens. 



This family is equivalent to the genus Spirifer of J. Sowerby 

 the father, the family Delthyridce, M'Coy, who gave some excel- 

 lent illustrations of the structure, and the Spiinferidce of King. 

 D^Orbigny proposed some genera under the names of Spiriferina, 

 Spirngera and Spirigerina, according to the direction of the axis 

 of the spiral cones, but it is doubtful if these genera are only 

 new names to those already established. 



The Spirifer of Sowerby, as reduced by M^Coy, and the Mar- 

 tinia of M^Coy, have the hinge as long or longer than the width 

 of the shell. In Atrypa, Dalman, and Athyris, M^Coy, it is 

 shorter and the shells oblong, rounder behind. 



According to the description of Mr. King, the genus Strigoce- 

 phalus would appear to form the passage between this and the 

 next family (Ann. Nat. Hist, xviii. 89). 



