198 DR. F. J. NORTH ON [vol. lxxvi, 



supporting-plates extend completely across the cavity of the valve, 

 and are divergent, enclosing between them an angle of about 35° ; 

 median septum well developed, reaching nearly to the level of the 

 area at the apex, becoming gradually lower, and disappearing about 

 half-way between the beak and the anterior margin. The septum 

 is, at its base, about as thick as the shell itself ; but it tapers rather 

 suddenly, and becomes very thin. Its apical portion is completely 

 embedded in the apical callosity, the structure of which is typical 

 of the genus, and has already been described. 



Remarks: — Shell-structure. — The majority of the Irish 

 specimens are silicifiecl, and afford no evidence as to the microscopic 

 structure of the shell ; but in suitably-preserved examples I have 

 found no conclusive evidence of punctas, neither when the shell 

 is examined with a hand-lens, nor when fragments are examined 

 under the microscope. The absence of puncta? cannot be due to 

 recrystallization, because the fibrous structure is clearly preserved. 

 The species under consideration cannot be readily distinguished 

 from SyringotJiijris on the minute shell-structure, but it cannot 

 possibly be confused with Spiriferiria. A lamellose Spirit erid f rom 

 the Bernician of Eedesdale (Northumberland), dealt with in a later 

 section of this paper, has a coarsely-punctate shell, and William 

 King, supposing it to be identical with the species under considera- 

 tion, assumed the existence of punctse in that species, which he, in 

 consequence, referred to Spiriferiua. 



Internal casts. — Owing to the development of the apical 

 callosity, an internal cast of the pedicle-valve of T. laminosa pos- 

 sesses distinctive characters. As will be seen from an examination 

 of text-fig. 4 h (p. 199) its profile differs considerably from that of 

 the external surface. The precise nature of the cast will be best 

 understood by reference to PI. XIII, fig. 12, where the central 

 conical portion (a) represents the cavity between the delthyrial sup- 

 porting-plates, and the lateral cones (b) the cavities on each side of 

 it; towards the centre of the figure are the casts (c) of the pit-like 

 depressions at the anterior end of the apical callosity, and between 

 them is the narrow fissure (7/) left by the unembedded portion of 

 the median septum. The significance of these structures will 

 be at once apparent if they are compared with PL XIII, fig. 15, 

 which illustrates the actual plate-system and apical callosity in a 

 specimen of T. subconi&a (Martin). They should be contrasted 

 with PI. XIII, fig. 14, which depicts an internal cast of a typical 

 Sjiiriferina : namely, Sp. ivalcotti. 



Internal casts such as those described were found by Principal 

 T. P. Sibly in dolomites of the Lower Caninia Zone in the Mitchel- 

 dean district, and they also occur in abundance in deep-red 

 dolomites at Tynant Quarry, Taff's Well, near Cardiff. 



Holot} T pe (of the emended species). — A specimen (brachial 

 valve) from Z. Hook Head (Wexford), in the Museum of Practical 

 Geology (Reg. No. 28425). 



