396 F. J. North — The genus Syringothyris. 



of the delthyriura in the Telotremata, but Hall & Clarke ' have 

 shown that ' pseudodeltidium ' was proposed by Bronn for the fused 

 condition of the deltidial plates that may occur in the Telotremata, 

 while the term deltidium was intended to be used for the simple 

 pedicle-secreted plate in the Protreraata and in some Neotremata. 

 Davidson used the terms indiscriminately, and referred to the plate 

 in Syringothyris and the Spirifers generally, sometimes as a deltidium 

 and sometimes as a pseudodeltidium. Hall & Clarke proposed to 

 replace the terms ' deltidial plates' and 'pseudodeltidium' by 'deltaria' 

 and * deltarium ' respectively, but this seems unnecessary if the 

 original definitions of deltidium and pseudodeltidium are adhered to. 



(2) The Delthijrial Supporting -plates. — Extending from the edges 

 of the delthyrial fissure into the interior of the pedicle valve are the 

 so-called * dental plates ' such as are developed in other brachiopods. 

 They are slightly divergent, and in the neighbourhood of the apex 

 reach the fl.oor of the valve ; but as the hinge-line is approached they 

 become very shallow, and are widely separated from the floor of the 

 valve. The function of these plates in the Orthotetids has been 

 discussed by Dr. Ivor Thomas,' who suggests that they should be 

 called delthyrial supporting-plates, since their almost complete 

 disappearance before the teeth are reached would make it appear that 

 their function is not so much to support the teeth, as to increase the 

 stability of the area where it is interrupted by the fissure. This 

 would apply with even greater force in the case of Syringothyris, 

 where the area is so high ; and the term ' delthyrial supporting- 

 plates ' will therefore be used in this paper in preference to * dental 

 plates *. 



(3) The Tuhe-hearing Plate, or Transverse Plate. — Connecting the 

 vertical delthyrial supporting-plates is a horizontal plate which 

 extends for from one-half to two-thirds of the distance from the apex 

 to the hinge-line. This plate is not the pseudodeltidium, but lies 

 below it. Its distal end is concave to the apex of the shell, and 

 attached to its lower surface is an incomplete tube which projects for 

 a short distance beyond the limits of the plate. On the upper surface 

 of the plate there is a median longitudinal ridge. In passing from 

 the apex towards the hinge-line the tube-bearing plate gradually 

 plunges beneath the level of the cardinal area. 



This tube-bearing plate developed between the delthyrial 

 supporting- plates of Syringothyris is the distinctive feature of the 

 genus. 



As already mentioned, the general characters of this plate and tube 

 have often been described, but their precise structure and relations 

 have not been understood. This is probably due to the state of 

 preservation of the shells examined. The structural details only 

 become really clear when the shell is preserved in a fine-grained 

 rock of such a colour that any shell structure which may be present 

 is clearly defined. Most of the specimens from Derbyshire and 

 Ireland are unfortunately filled with crystalline calcite, which tends 

 to obscure the internal characters of the shell. 



1 Hall & Clarke, 1894, pp. 327-8. 

 - Ivor Thomas, 1910, pp. 100-1. 



