F. J. North — The genus Syringothyris. 397 



4. Detailed Account of the Structure of the Tube-beakinq 

 Plate (Transverse Plate) in a specimen of Sybingothybis 

 AFF. cABTEBi (Hall). (Plate XII.) 



The specimen here described was obtained by Professor T. F. Sibly 

 from the Upper Cleistopora Zone (Kg) of the Post Office Quarry at 

 Howie Hill, in the Forest of Dean area, Gloucestershire. It is of 

 a type that is common in the Upper Cleistopora Zone and in the 

 Zaphrentis Zone of the South- Western Province, and which, as 

 mentioned by Dr. Vaughan,^ has broad flat ribs on the pedicle valve. 

 This type of shell differs in certain respects (height and convexity of 

 area, etc.) from the Derbyshire and Kildare specimens, and is more 

 nearly related to Syringothyris carteri (Hall) than to S. cuspidata 

 (Martin). The consideration of these differences is, however, best 

 deferred until a future paper, when the species of Syringothyris will 

 be dealt with in detail. 



Owing to the nature of the matrix, a fine-grained yellow limestone, 

 and the excellent preservation of the shell substance, the specimen 

 was peculiarly suitable for study of the internal characters. 



The pedicle valve of the shell was rubbed down in a plane at right 

 angles to the area, and sections drawn at intervals of about half 

 a millimetre. The beak of the valve was slightly damaged, and the 

 first satisfactory section was obtained at a distance of 4 mm. from 

 the apex. The following sections are selected as illustrating the 

 more important stages. (See Plate XII, Figs. 1-8.) 



Figs. 1 and \a. Distance from the apex 4 mm. In this section 

 there are two strong divergent plates crossing the valve from the 

 cardinal area to the floor. Each plate is divided longitudinally into 

 halves by a distinct line. The outside moiety (the true delthyrial 

 supporting-plate) in each case is continuous with the shell substance 

 of the cardinal area, while the inside portion curves over at the top, 

 and meeting its fellow from the opposite side forms an arch-shaped 

 plate (the transverse plate) between the delthyrial supporting-plates, 

 and which when seen from above appears as a septum connecting 

 those plates. 



Figs. 2 and 2a. Distance from the apex 6' mm. Here the sides of 

 the arch-shaped plate are thinner than the delthyrial supporting- 

 plates, which are themselves thinner than in the previous section. 

 In the transverse portion of the arch there is a circular mark which 

 represents the cavity of a tube, filled and obscured by shelly matter 

 during the subsequent growth of the plate. On the upper surface 

 of the plate there is a slight median ridge, also seen in the 

 subsequent sections. 



Figs. 3 and Za. Distance from the apex 1 1 mm. Between this 

 section and the previous one, the delthyrial supporting-plates have 

 become thinner at their bases, until in the present section one of 

 them fails to reach the floor of the valve. The limbs of the arch- 

 shaped plate are shorter than aiid thin out against the delthyrial 

 supporting-plates. The outline of the tube or syrinx is very distinct, 

 but its cavity is still filled with shelly matter. 



1 Vaughan, Q.J.G.S., vol. Ixi, p. 301, 1905. 



