164 DR. F. J. NORTH ON [vol. lxXV'i, 



Apical angle. — The angle enclosed at the apex of the cardinal area. 



Delthyrial angle. — The angle enclosed by the sides of the delthyrial 

 fissure. 



Cardinal extremities. — The terminations of the hinge-line. 



Lateral slopes. — Those portions of the surface of both valves which lie on 

 either side of the fold in the brachial valve and the sinus in the pedicle-valve. 



Costae. — The components of the radial ornament of the shell of Syringe- 

 thyris and other brachiopoda have usually been termed 'ribs' and, less 

 frequently, ' costae.' In order to assist in securing uniformity in terminology, 

 it is proposed to adopt the suggestion of the late Dr. Ivor Thomas, and to use 

 only the term ' costse ' in this connexion. As that author pointed out, 1 ' the 

 radially disposed features hitherto called ribs appear to correspond to the 

 " spirals " of gasteropoda,' and certainly not to their ribs, which are con- 

 centric in their arrangement.' On these grounds he used the term ' ribs ' for 

 the concentric rugae of the Producti, and ' costas ' for the components of the 

 radial ornament. 



II. Terms applied to Internal Characters. 



Delthyrial supporting-plates .—The delthyrial sxipporting-plates are a pair 

 of lamellae in the pedicle-valve, which project from the edges of the delthy- 

 rium into the interior of the shell. These plates were formerly termed ' dental 

 plates,' because they were supposed to exist for the support of the teeth ; but 

 the late Dr. Ivor Thomas, discussing their function in Orthotetes, proposed 

 the term 2 ' delthyrial supporting-plates ' : and since, in the shells under con- 

 sideration, the plates have almost disappeared long before they reach the 

 hinge-line, it would seem that their primary function is not to support the 

 teeth, but to strengthen and support the thin shell of the cardinal area where 

 it is interrupted by the delthyrial fissure. 



Median septum. — Vertical shelly septa may exist in the longitudinal median 

 plane of either valve ; and where, in order to avoid confusion, it is necessary 

 to distinguish between them, the terms ' ventral median septum ' and ' dorsal 

 median septum ' may be used. 



Ventral median septum. — In many brachiopoda the muscular scars in the 

 pedicle or ventral valve are bisected by a distinct ridge, which in some genera 

 becomes an elevated shelly plate or septum rising into the cavity of the shell, 

 and extending farther towards the anterior margin than do the delthyrial 

 supporting-plates on either side of it (see text-fig. 1, m, n, o, p, p. 166). The 

 function of the septum was presumably to afford additional surface for the 

 attachment of the adductor muscles, the scars of which extend over its 

 sides. 3 The presence or absence of a ventral median septum, and its nature 

 when present, are important taxonomic characters in the group of shells 

 under consideration. 



Dorsal median septum. — This is never more than a low ridge in the genera 

 here discussed, and it is of no value in discriminating between species. 



Apical callosity. — There is frequently a tendency to an excessive deposition 

 of shelly matter in the apical portion of the pedicle-valve, which becomes 

 greatly thickened, or even completely solidified. Hall & Clarke described 

 this character as the ' apical callosity,' and it was referred to by the late 

 Dr. A. Vaughan as the ' rostral callus.' 



1 'British Carboniferous Producti' Mem.' Geol. Surv. Gt. Britain, Palae- 

 ontology, vol. i, pt. 4 (1914) pp. 225-26. 



2 ' The British Carboniferous Orthotetinae ' Mem. Geol. Surv. Gt. Britain, 

 Palaeontology, vol. i, pt. 2 (1910) pp. 100-101. 



3 J. Hall & J. M. Clarke [14] p. 53, fig. 42. The occurrence of the muscular 

 scars on the sides of the ventral median septum has been recorded in Spiri- 



ferina vxclcotti by Hall & Clarke, op. cit., and in Sp. octoplicata by W. King. 

 'Monogr. Perm. Foss.' Pal. Soc. 1850, p. 68. 



