DAVIDSON — PAL JSONTOLOGICAL NOTES ON THE BRACHIOPODA. Ill 
neuces (w), termed " oral processes " by Mr. Woodward, and these (z) 
I have found to correspond almost exactly with the centre of the sub- 
spiral hollows in the ventral valve (l). It is therefore highly probable 
that at least a portion of the arm was spirally coiled, and occupied the 
spaee existing between and all round the conical testaceous projec- 
tions (z) described above, as occurring in the dorsal valve, and the 
hollow [1) in the ventral one,* in the same manner as we have shown 
to have been the case with Davidsonia,\ but with this difference, that 
in Proclucta there exists a conical elevation in the dorsal valve, and a 
coiTesponding hollow in the ventral one ; while the very reverse was 
the case in Davidsonia and Straphomena depressa or rhomboidalis of 
Dalman (PI. IV. fig. 13, l). From this I would surmise that the arms in 
the Productidce were differently disposed to those in the Davidsonidce, 
and at least some of the Sti ophomenidce ; that is to say, the whole or 
a portion of the arm formed a few vertical convolutions directed 
towards the bottom of the dorsal valve in Davidsonia and Strophomena 
depressa, while in Producta they were, on the contrary, directed towards 
the bottom of the hollows of the ventral one. I also consider that 
those deep conical hollows observable in the interior of the enormously 
thickened ventral valve of Producta Jmmerosa, Sow., as seen in relief 
upon the internal cast (PI. IV. figs. 3, l), to have been occupied by 
the arms. Such is the only reliable evidence that can be offered at 
present as to the form and position of the arms ; but it is necessary 
here to mention that in his excellent work on the Petschoraland (pub- 
lished in 1846), Count Alex, von Keyserling has expressed an opinion 
that the so-termed uniform impressions or callosities (PI. III. fig. 2, 
and PI. IV. figs. 5, 7, 12, x) observable in the dorsal valve of all the 
Productidce, were the probable supports of the oral arms ; which view 
was at a later period reproduced by Mr. Howse, who, moreover, 
assimilates those impressions to the ridges supporting the arms in the 
interior of Argiope and Thecidium. Mr. Hancock is disposed to accept 
* In a letter I had the pleasure of receiving from Count Keyserling, it is stated, 
that, " if in the ventral valve of Producta gigantea, and some few others, we see 
the indication of obscure spiral depressions, this may be due to the unattached 
portion of the arms ; but that we perceive no similar hollows in the same valve of 
the greater number of species ;" but it should also be remembered that we cannot 
expect to find hollows in those forms in which the valve was too thin to admit 
of similar depressions. 
t The Geologist, Vol. I., PI. XII. figs. 33, 34. 
