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 bo 
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 Terelratulidcc, wherein the 
median arm alone was spirally coiled. In Rynchondla, Strophomena, 
and Productus, the arms were spirally coiled, but unsupported by any 
testaceous appendages ; while in Spirifera, Athyria, 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 Anoplothcca, the 
two spirally-coiled lamellffi 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 Ehynchonellidce, 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 
Terebratulidae it has been ascertained that, in some species, the 
oral arms were supported at their origin only, as in Terehratula vitrca, 
T. caput-serpentis, &c., while in other genera the ribband-shaped lamellte 
or loop extended for a considerable distance into the interior of the 
shell, as in Waldhcimia, Terehratella, ^,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 
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