Dr. O. F. Mattheio — The Oldest Siphonotreta. 69 



out, while the adjoining shell-layers are weathered away. The shell 

 of this species was thin during the Schizambonal stage, but thickened 

 rapidly by the accretion of layers within the shell during the Sipho- 

 notretal stage. As, however, the parts of these layers around the 

 tube were not pei'fectly calcified, that organ in exfoliated shells 

 stands out and remains as a projecting tube, after the layers of shell, 

 as above remarked, are weathered away. 



In an exfoliated shell, therefore, we have a siphon projecting from 

 what seems to be the inner side of the shell, and simulating the 

 figures given of the interior of the ventral valve of Siphonotreta 

 unguiculata, Eichwald."^ And the resemblance to that species in 

 other respects is significant, for, in the first place, the passage for 

 the pedicle in the new form diminishes in size from that which it 

 had at the close of the Schizambonal stage, until maturity ; and it 

 will be noted that in the species of Siphonotreta named above, it is 

 said that the internal tube diminishes in size as it passes inward.^ 



It is also stated that in Siphonotreta unguiculata the muscle-marks 

 are very near the internal opening of the tube. This is not notice- 

 ably the case with the new form when the interior of the shell is 

 completely preserved ; but in exfoliated examples where the siphoual 

 tube is exposed, the muscle-markings on the shell-layers remaining, 

 being those of the early Siphonotretal condition, are much nearer the 

 siphon, and so are like those accredited to the species above named. 



The new form has no exsert siphon showing on the inner surface 

 of the shell, and it will easily be seen that such a projecting tube 

 would involve anatomical conditions different from those of most of 

 the early Neotrematous Brachiopoda. It would appear that a siphon 

 projecting inside the shell is not an invariable characteristic of 

 Siphonotreta, for Davidson figures S. unguiculata with a scarcely 

 projecting siphon,^ and De Verneuil shows the inside of an umbo of 

 S. verrucosa, De V., in which the opening for the pedicle lies in 

 a little saucer-shaped hollow, as in our species.^ 



In some species referred to Siphonotreta, the pedicle passage opens 

 outward just behind the beak ; in others a channel extends along the 

 back of the shell for some distance, and then a hole gives passage to 

 the interior of the shell, there being no siphon or tube. These latter 

 have been divided off by Walcott as the genus Schizambon. Since, 

 however, the St. John species agrees exactly neither with this section 

 nor the other, but in its pedicle passage combines the characters of 

 both, it is necessary to establish for it a separate place. Linking 

 together as it does the genera of De Verneuil and Walcott, it would 

 seem proper to regard both Schizambon and the new form as sub- 

 genera of Siphonotreta, and with this view the author would propose 

 for the new form the name Pkotosiphon, for which the above 

 remarks will give the essential characters. 



1 See "Manual of the MoUusca," S. P. Woodward, London, 1875, p. 390, 

 fig. 201. 



2 Hall and Clarke, " Genera of Palaeozoic Brachiopoda," Albany, 1892, p. 110. 



3 Geol. Mag., London, 1877, PL II, Figs. 9, 11. See also Hall and Clarke, 

 "Genera of Palaeozoic Brachiopoda," pi. iv, fig. 25. 



* " Eussia and the Ural Mountains," Paris, 1845, yoI. ii, pi. i, fig. lid. 



