172 DE. F. J. Is'ORTH OX [vol. Ixxvi, 



of punctae, to the remainder of the shell. The central region, 

 however, which is bisected by the delthyrium is impunctate, and in 

 addition to the horizontal lines of growth, is marked by distinct, 

 hut irregularly-spaced, vertical stria? (see fig. 2, p. 170). l 



The vertical strise are the external indications of certain internal 

 peculiarities of shell-structure. In partly-exfoliated specimens 

 this peculiarity evinces itself in a series of trough-like depressions, 

 which suggest that the shell-tissue of the cardinal area was tra- 

 versed by numerous canals, perpendicular to the hinge-line. This, 

 however, does not appear to have been the case, as no trace of a 

 canal-system is apparent when thin sections of the area are 

 examined under the microscope, and there is never any in tilling 

 by darker matrix such as is frequently the case with the tubular 

 punctations of the shell of Spiriferina walcotti. The strise are 

 connected with a series of denticles along the hinge of the pedicle- 

 valve, and articulate with a row of pits in the hinge-line of the 

 opposite valve. 



Davidson suggested that the function of the denticles was to 

 steady the valves, especially when the hinge-line was long and the 

 teeth not very large. 2 As the shell increased in size by the 

 peripheral addition of shelly tissue, and the denticles receded from 

 the apex, their course was marked by a series of lines, approximately 

 parallel, and perpendicular to the hinge-line. 



In a discussion of the origin of the vertical strhe, J. Young 

 suggested that they were due to the presence in the hinge-line of 

 the living animal of fibres of aragonite, which 



' being harder than the ordinary calcite of the shell went to the formation of 

 the row of denticles.' 



If this were the case, the different degrees of resistance offered by 

 the two minerals to subsequent alteration would account for the 

 appearance of the canals in some specimens, for aragonite, although 

 harder than calcite, is more soluble than that mineral. 



The vertical strise are only to be seen in specimens which have 

 undergone but little mineralogical change, and they are often 

 completely obliterated by recrystallization or exfoliation. The 

 denticles are very rarely seen. Occasionally, as in the case of the 

 specimen upon which Young's observations were based, the two 

 valves are separated in such a way as to expose them clearly. In 

 recrystallized specimens they have quite disappeared; but, according 

 to Young, in some cases they are rendered visible by scraping and 

 etching with acid. In this way he proved their existence in 



1 Dr. Ivor Thomas described in some of the Orthotetinae : a separate areal 

 portion in the form of a raised triangular platform, marked off around- the 

 delthyrium. This secondary area is distinguished by vertical striations in 

 contrast to the horizontal striations of the larger surrounding primary area.' 

 In Syringofhyris, however, the plane of the area is not interrupted by the 

 central portion. 



2 J. Young, ' On the Denticulated Structure of the Hinge-Line of Spirifera 

 trigovalis' Geol. Mag. 1884, pp. 18-20. 



