398 F. J. North — The genus Syringothyris. 



Figs. 4 and 4.a. Distance from the apex 14 ram. The delthyrial 

 supporting-plates are here very short. In the transverse plate the 

 cavity of the tube and its inferior opening are very clear. This 

 section, in which the transverse plate and split tube are perfectly 

 developed, may be taken as a type-section of the essential characters of 

 the genus, and constitutes a standard to which sections of other 

 specimens can be referred. 



Figs. 5 and 5«. Distance from the apex 15 mm. In the sections 

 following that shown in Fig. 3, the limbs of the arch become shorter 

 and shorter until they are mere flatteuings of the edges of the trans- 

 verse plate. The plate itself and the wall of the tube become 

 thinner. The conditions at this stage are shown in Fig. 5. 



Figs. 6 and 6a, 7 and la. Distance from the apex 16 and 17 ram. 

 respectively. The transverse plate eventually disappears, except for 

 two small portions, one applied to each of the delthyrial supporting- 

 plates, leaving tlie syrinx quite free as in Fig. 6. At the same time 

 the syrinx becomes smaller and its walls thinner, while its roof 

 assumes a foLied aspect. In Fig. 7 the lateral portions of the trans- 

 verse plate have disappeared, as have also the side walls of the syrinx, 

 leaving only the roof of the syrinx, continuing as a ridged and folded 

 lamella which becomes gradually smaller and finally disappears. (In 

 this specimen, at a distance of 19 mm. from the apex of the shell.) 



In the earlier sections (Figs. 1-3) there is a low ridge on the 

 floor of the valve between the delthyrial supporting-plates. This 

 ridge or crest divides the anterior portion of the muscular impressions, 

 and resembles the median crest which occurs in Spirifer, Productus, 

 and many other brachiopods, but contrasts very strongly with the 

 elevated median septum which occurs in a similar position in 

 Spiriferina. 



In a band of dolomite in the Lower Zaphrentis Zone (Zj^), exposed 

 in the Cement Works Quarry at Mitcheldean, Glos, internal casts 

 of Syringothyris of the type already described are abundant, and 

 many specimens have been collected by Professor Sibly. The shell 

 substance has been removed by solution, but the characters of the 

 shell are very clearly shown by the matrix, with which the valve 

 was filled. The delthyrial supporting-plates and the transverse plate 

 are represented by space, and the cavity of the syrinx by a rod 

 (r, Figs. 8, 8«) lying in a groove on the surface of the matrix that 

 filled the chamber between the delthyrial supporting-plates. The 

 lower surface of the rod is connected with the floor of the groove by 

 a narrow plate representing the slit in the syrinx. The muscular 

 impressions on the floor of the pedicle valve and the crest which 

 separates them are clearly defined in these casts. It was from 

 a similarly preserved specimen that Davidson inferred the presence 

 of a foramen in tlie pseudodeltidium of Syringothyris cuspidata 

 (see p. 395). 



We have seen that the transverse plate of Syringothyris arises in 

 the apex of the shell as an arch-shaped plate, between and applied 

 to the delthyrial supporting-plates, but not as an integral part of 

 those plates. The syrinx occurs on the lower surface of the plate, 



