DAVIDSON— ON SOME FOSSIL BRACHIOPODA. 



411 



Rhynchonella psittacea divided into two largely-developed arms with a 

 single row of cirri, and without any testaceous support, but that the 

 reverse was probably realised in Spirifera, where the median arm is sup- 

 posed not to have existed ; and that the cirri were exclusively borne by 

 calcareous spiral appendages, divided into two halves, independent of 

 each other, and completely symmetrical ; but further research will be 

 required before this important point can be completely determined. 



The oral arms appear to have differed much in position and detail. They 

 were variously coiled in all the articulated genera, with the exception 

 of those that compose the great family Terehratulidce, wherein the 

 median arm alone was spirally coiled. In ET/nchonella, Stropliomenat 

 and Produdus, the arms were spirally coiled, but unsupported by any 

 testaceous appendages ; while in Spirifera, Athyris, and their sub- 

 genera, the oral arms were supported or borne by two long ribbon - 

 shaped lamella? spirally coiled, and with their extremities directed 

 outwards. In Atrypa, Davidsonia, Koninckina, and Anoplotheca, the 

 two spirally-coiled lamella? were vertical and directed towards the 

 bottom of the valve, or horizontally disposed, each coil being nearly on a 

 level with the bottom of the valve. Their principal conditions were, 

 therefore, assumed by these spirally-coiled arms, and by their support- 

 ing processes ; and a very important question has arisen, — that is to say, 

 whether we can or ought to place in the same family, animals in which 

 the spiral arms were free, and those in which they were borne or sup- 

 ported by calcified spiral lamellae } Mr. S. P. Woodward is of opinion 

 that Atrypa should be removed from among the Spiriferidce, and 

 be added to the Rhynchonellidce, considering the calcification of the 

 oral supports to be a character of uncertain value? And this view is 

 certainly deserving of some further consideration, although I felt dis- 

 posed, while composing my General Introduction, to place Dalman's 

 Atrypa in the same family with Spirifera and Athyris. Among the 

 Terebratulida) it has been ascertained that, in some species, the 

 oral arms were supported at their origin only, as in Terebratula vitrea, 

 T. caput-serpentis, kc, while in other genera the ribband-shaped lamellse 

 or loop extended for a considerable distance into the interior of the 

 shell, as in Waldheimia, Terebratella, ^c. ; but, although the shape and 

 extent of the calcareous processes may have served as a good character 

 for the grouping of similar forms into the same genus, still their presence 

 or absence does not appear to have very materially interfered with the 



2 H 2 



