ORDER II NEOTREMATA 309 
Siphonotreta, de Vern (Fig. 511). Shell elongate-oval, moderately convex. 
Pedicle opening at the apex, and communicating with the interior of the 
shell by a tubular canal. 
Cardinal area and del- 
tidium not developed. 
Shell ornamented with 
concentric lines and 
ridges; the epidermal 
layer bearing hollow 
spines, though rarely 
preserved. Layers be- Siphonotreta wnguiculata, Eichw. Ordovician; St. Petersburg. 1/3. 
Beery che! epidermis 4,2 tntevey of venta! seal ame eave, tects 6, 2, Wor 
punctured by radiating 
and branching tubules. Ordovician and Silurian; Europe and (?) North 
America. 
Schizambon, Walcott (Schizambonia, Oehlert). Small depressed Siphono- 
tretae without tubular pedicle canal. Ordovician ; America and Russia. 
Trematobolus, Matthew.  Siphonotretae with rudimentary articulation. 
Middle Cambrian ; New Brunswick. 

Superfamily 2. DISCINACEA. Waagen. 
Neotremata with phosphatic shells, a listrium, but no deltidium. Dorsal pro- 
tequlum usually sub-central. Cambrian to Recent. 
Family 1. Trematidae. Schuchert. 
Primitive Discinacea, in which the posterior margin of the ventral valve has a 
triangular pedicle notch throughout life. A listrium usually present. Cambrian to 
Carboniferous. 
Discinolepis, Waagen. Small, nearly flat, inequivalved shells. Pedicle 
notch very small. Characters very primitive. Lower Cambrian ; India. 
Trematis, Sharpe (Orbicella, Orb.) Ventral valve unevenly convex, 
more or less depressed over the posterior region. Pedicle fissure large, ex- 
tending from the apex to the posterior margin. _ Dorsal valve evenly convex, 
and sometimes with incurved beak ; posterior margin much thickened, and 
broadly grooved for the passage of the pedicle. Surface of both valves 
covered with punctures or small pittings arranged either in quincunx 
or in radiating rows. Ordovician and (?) Silurian; North America and 
(2?) Europe. 
Schizocrania, Hall and Whitfield. Ventral valve flat or concave, smaller 
than the dorsal, and bearing a deep and very broad triangular pedicle 
notch, which extends from just behind the beak to the posterior margin. 
Apex of notch occupied by a triangular plate, or listrium. Surface 
marked by concentric growth lines; no muscular impressions visible on 
the interior. Dorsal valve more or less convex, with beak marginal. 
External surface radially striated. On the interior, a low median ridge 
extends from the apex to beyond the centre of the valve; posterior adductor 
muscles strong; the anterior ones faint. Ordovician to Devonian; North 
America, 
