part 2] SVRIXGOTHi'llIS AND SPIIUFERIXA. 171 



causes a wide rounded wave in the anterior margin. Except for a 

 slight median depression on the fold, extending for a short distance 

 from the beak of the brachial valve, the fold and sinus are smooth. 1 



The lateral slopes of each valve are typically ornamented by 

 radial costa?, decreasing gradually in size towards the cardinal 

 -extremities. The costa j are regularly broad and depressed, and 

 iire separated by narrow, shallow furrows ; they are always simple, 

 and originate at the hinge-line, the last four or rive on either side 

 being small and inconspicuous. They are frequently obsolete, 

 ■especially in the pedicle-valve, the lateral slopes of which may be 

 quite smooth. 



The costa; are crossed at irregular intervals by inconspicuous 

 lines of growth, which are more closely crowded towards the margins 

 •of the valves. The external surface of the shell in suitably- 

 preserved specimens is marked by a minute textile-like ornamen- 

 tation, and the shell-substance is minutely punctate. 



Internal structures: Pedicle- valve. — There is a pair of 

 delthyrial supporting-plates which extend completely across the 

 cavity of the valve, to the floor of which they are attached for 

 about one-third of its length. 



The transverse plate extends for from a third to half of the 

 distance from the apex to the hinge-line, and has on its lower 

 surface the split tube or syrinx. 



' The transverse plate arises in the apex of the shell as anarch-shaped plate 

 between, and applied to, the delthyrial supporting-plates .... The syrinx 

 occurs on the lower surface of the plate, and is formed by two nearly- 

 parallel lamella?, the free ends of which curve towards one another. As the 

 distance from the apex of the shell increases, the lateral portions of the arch 

 become thinner and shorter, until only its roof persists as the tube-bearing - or 

 transverse plate — a slightly convex plate down the centre of which is a 

 median ridge on the upper surface, and a slit tube or syrinx on the lower 

 surface. The transverse plate itself then dies out. Its distal end is concave 

 towards the apex of the valve, and the syrinx projects for a short distance 

 beyond it ' 2 (see figs. 1 a-lc, p. 166). 



The earlier-formed portion of the syrinx tends to be obscured by 

 the deposition of shelly matter (an apical callosity) during the 

 growth of the shell (see tig. la, p. 166). 



Brachial valve. — There is a low median septum extending 

 for about half the length of the valve. Spiral coils are known 

 to be present, but in the specimen in which they were developed 

 by the Rev. Norman Glass, they were not in situ, and the nature 

 of the primary lamellae and jugum is, as yet, unknown. 



The threefold division of the area. — In well-preserved 

 specimens the area is clearly divided into three triangular portions, 

 of which the two lateral portions are marked by horizontal striae 

 and are similar in intimate structure, namely, in the possession 



1 Two species with plicated fold and sinus have been referred to Syringo- 

 ihyris, but in neither case has the existence of a syrinx been proved. These 

 are fully discussed elsewhere (see p. 189). 



2 North [25] pp. 398-99. 



