F. J. North — The genus Syringothyris. 399 



and is formed by two nearly parallel lamellae, the free ends of which 

 curve towards one another. As the distance from the apex of the 

 shell increases, the lateral portions of the arch become thinner and 

 shorter until only its roof persists as the tube-bearing or transverse 

 plate — a slightly convex plate, down the centre of which is a median 

 ridge on the upper surface and a slit tube or syrinx on the lower 

 surface. The transverse plate itself then dies out. Its distal end 

 is concave towards the apex of the valve, and the syrinx projects for 

 a short distance beyond it. The slit on the ventral surface of the 

 tube tends to close, and the tube itself to be obscured by the further 

 deposition of shelly matter on the earlier-formed portion of the 

 transverse plate. In discussing the homology of the plate, Winchell ^ 

 stated that in many Spirifers there is "an indication of a longitudinal 

 folding of the dental plates which may produce on one side or the 

 other a lanainar process", and he suggested that the plate of 

 Syringothyris may be of this nature, but in view of the sections that 

 have been described this view appears to be untenable. 



5. Relation BETWEEisr Syringothyris and certain Spikifees ; and 

 THE Origin oe Syringothyris in Noeth America. 



There is, developed between the delthyrial supporting-plates of 

 certain Spirifers, a short transverse plate resembling in position and 

 mode of origin the tube-bearing plate of Syringothyris ; and the 

 following sections (Plate XII, Figs. 9-11) across the beak of the 

 pedicle valve of Spirifer duplicicosta, Phillips, from the Carboniferous 

 Limestone (Visean) of Park Hill, Derbyshire, may be compared with 

 those of Syringothyris (Plate XII, Figs. 1-7). 



Fig. 9. Distance from the apex 3 mm. This section should be 

 compared with Fig. 1 ; at this stage the structure of the two shells is 

 essentially the same. 



Fig. 10. Distance from the apex 5 mm. This corresponds in 

 position with. Fig. 4, and it will be seen that although the relation 

 between the transverse plate and the delthyrial supporting-plate is 

 the same in each case, there is no trace of a tube in the present 

 section. 



Fig. 11. Distance from the apex 8 mm. This section corresponds 

 to a stage between Figs. 5 and 6. The plate is incomplete in the 

 centre owing to its concave extremity as in Syringothyris, and in 

 a further section, 11 mm. from the apex, it does not appear at all. 



A similar transverse plate was observed by the writer in a large 

 specimen of Spirifer striata (Martin) from Kildare. King^ claimed 

 to have seen in a specimen of S. striata a transverse plate in which 

 there was actually a tabular canal, but this has not been verified. 



Origin of Syringothyris in North America. — The plate just 

 described as occurring in Spirifer duplicicosta, Phillips, and S. striata 

 (Martin) is also found in many Devonian Spirifers of North America, 

 and has been described and figured by Hall & Clarke,^ who call it 

 a delthyrial callosity. In North America there appears to have been 



1 Wincliell, 1863, p. 7. 



2 King; 1868, p. 18, and pi. ii, fig. 25. 



^ Hall & Clarke, 1894, pis. xxiii, xxiv, etc. 



