Vol. 67.] TEE AVONIAN OF BURRINGTON COMBE. 383 



(2) A large geniculate (Leptsenoid) form with a flat cardinal 

 region. 



The form that has just been described has a strongly convex beak 

 and beak-region ; the present form is equally large and even more 

 massive, but has a flat cardinal region occupying half the valve, 

 the rest of the valve being gently flexed. The shell is markedly 

 transverse. 



Although the immediate ancestors of this form are unknown, it 

 is not improbable that Chonetes cf. laguessiana is an early ancestor, 

 for reduction in size and massiveness by parallel development with 

 the first-described form would result in just such an ancestor as 

 Ch. cf. laguessiana. 



Occurrence. — The development of this Leptsenoid form was 

 probably carried out in the Pembroke area, where it reaches its 

 greatest abundance ; it becomes less abundant when traced east- 

 wards, and, in Belgium, the discovery of a rare example was 

 recorded and described by M. P. Destinez. 1 



At Burrington this form is a rare associate of the Productoid 

 form described above. In other parts of the South- Western 

 Province it is, as already stated, much commoner, and reaches its 

 acme in C„. 



The Eelation between Syringothyris and Spiriferina. 

 Sykingothyeis laminosa (auctt.) non (M ; Coy). (PI. XXXI, fig. 8.) 



Spiriferina laminosa cle Kon., Ann. Mus. Eoy. Hist. Nat. Belg. vol. xiv, 

 pt. 6 (1887) pp. 103-105 & pi. xxii, figs. 44-50, pi. xxx, figs. 30-31. 



Range. — Z 2 and C^ 



External characters. — The general aspect is that of a 

 large Spiriferina, with a well-developed area and coarse ribs. As 

 in Spiriferina, there is a delthyrial callus extending for a short 

 distance only from the apex. The shell is punctate, as in both 

 Spiriferina and Syringothyris. 



Internal characters. — Wholly or partly buried within 

 this callus is an infilled syrinx that can be seen half projecting 

 along the middle line of the callus and, in weathered specimens, 

 is represented at the apex by a short narrow rod. 



A cross-section near the beak shows the infilled syrinx and the 

 two dental plates ; at a little distance below the beak the strong 

 mesial septum begins to appear. 



In these two characters : — 



(1) The presence of a syrinx, 



(2) The late development of a mesial septum, 



consists the claim of the species to belong to Syringothyris. 



Comparison with the genotype of Spiriferina. — 

 A cross-section of Spiriferina rostrata, from the Middle Lias, at 

 the beak shows the absence of a syrinx and the immediate appear- 

 ance of the mesial septum. 



1 Ann. Soc. geol. Belg. vol. xxix (1901-1902) Bull. p. 106. 



