ORDER ii NEOTREMATA 



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 

 tidiuni not developed. 

 Shell ornament (M! with 

 concentric lines and 

 ridges; the epidermal 

 layer bearing hollow 



spines, though rarely K|(: M1 



preserved. Layers be- .SV/.AO^^V^ wngvAovktta, i:\i-\\\\-. ordovu-ian ; st. Petersburg. >/,. 



noatli tlio nrkirlormia A > B > Interior of v.-ntral and dorsal valves, respectively. C, I), Worn 



neath the epidermis ex ' ter ' ior of ghcll . ventral antl dorgal a si,.'i-ts. 



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- 

 fegulum 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. 



Discindepis, Waagen. Small, nearly flat, inequivalved shells. Pedicle 

 notch very small. Characters very primitive. Lower Cambrian ; India. 



Trematis, Sharpe (Orbicella, d'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 

 (?) 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, 



