niiAcmopoDA. 



of the shell, and the substiincr of tlu- valves is not pierecd with 

 mimite perl'orations, thoufjli the surface is sometimes spiiiulose, 

 the spines beino; only formed on the edge of the shell w hile it is 

 beiu"; increased in size. 



'a 



Order III. Sclerohrachia. 



The oral arms su|)port a shelly band arisinf; from the hinder 

 or cardinal eilge of the ventral valve. 



Fam. 1. Spiriferidcs. 



The oral arms veiT larjjely developed and supported the whole 

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

 plate. 



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

 supports of the arms are generally preser\'ed, and may be dis- 

 covered 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. Sowerbv 

 the father, the family Delthyridm, M'Coy, who gave some ex- 

 cellent illustrations of the structure and the Spiriferidae of King. 

 D'Orbigny proposed some genera under the names of Spirifer ina, 

 Spirigera 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. 



Accorchng to the description of Mr. King, the genus Strigo- 

 cephalus would appear to form the passage between this and the 

 ne.\t family [Ann. Nat. Hist, xviii. 89). 



Fara. 2. RhynchonelUdce . 



The oral arms are elongate, fleshy, supported at the base bv 

 two short, hard, diverging shelly laminae arising from the hinge- 

 margin of the ventral valve. 



They are easily known from the TerebratulidcB by the cavity of 

 the shell being without shelly plates, its substance not perforated, 

 and its surface being generally radiately plaited. 



Only one species, T. psittacea, is known in the recent state ; 

 its animal has been described by Professor Owen. The family 

 is equivalent to the plaited Terebratula of the elder James Sowerby 

 and Von Buch, the non-perforated Terebratula of Carpenter, the 

 genus Hypothyris of Phillips, and part of the family Terebratu- 

 Udce of King. It contains the genus Rhynchonella of Fischer and 

 D'Orbigny — Hypothyris of Phillips ; Camerophoria, King ; 



B 5 



