172 BRACHIOPODA 



area and a distinct groove and slit for the peduncle. Cambrian to 

 Ordovician. Ex. L. davisi, Lingula Flags. 



ORDER II. NEOTREMATA 



The peduncle-opening is confined to the ventral valve. 

 In the lower types the opening is in the form of a slit 

 at the margin of the valve ; but in the higher forms it 

 is completely surrounded by shell, and is often near the 

 centre of the valve, in which case the peduncle passes out 

 at right angles to the plane of junction of the two valves. 

 A pseudo-deltidium is sometimes present. E.g. Distinct. 



Siphonotreta. Shell elongate-oval, biconvex, inequivalve, 

 with spines on the surface. Hinge-line curved ; no area. Ventral 

 valve the more convex, with a prominent, straight umbo, having a 

 foramen at its apex continued as a tube to the interior of the valve. 

 Dorsal valve less convex, with umbo at the margin. Muscular 

 impressions near the hinge- line in both valves. Ordovician and 

 Silurian. Ex. S. micula, Llandeilo. 



Discina (group). Shell conrposed partly of chitinous material ; 

 sub-orbicular, or sub-elliptical, surface smooth or covered with striae 

 of growth. Valves more or less conical, the summits of both sub- 

 central or sub-posterior. Peduncle-opening placed either near 

 the summit of the ventral valve or a little behind it. Four 

 adductor impressions. Cambrian to present day. Discina, in the 

 wide sense, as defined above, includes the three genera Discina 

 (restricted), Discinisca, and Orbiculoidea. 



Discina (restricted). Both valves convex. Peduncle-opening 

 small, near the middle of the valve externally, passing through the 

 shell obliquely forwards. The only species definitely known is 

 D. striata, Recent. 



Discinisca. Ventral valve flattened ; behind the apex is a 

 disc, which is depressed externally and interrupts the continuity of 

 the lines of growth. The disc is perforated for the peduncle by a 

 fissure which passes directly, not obliquely, through it. Tertiary 

 and living ; perhaps Mesozoic. Ex. D. lamellosa, Recent. 



