CLASS II BEACHIOPODA 377 



Linnarssonella Walcott. Middle Cambrian ; North America. 



Discinopsis Matthew. Has marked ventral vascular sinuses. Middle 

 Cambrian ; New Brunswick. 



(1) Mesotrefa Kutorga. Ventral valve spinose. Basal Ordovician ; 

 Esthonia. 



Superfamily 3. DISCINACEA Waagen. 



Sjiecialised Neotremata with phosphatic shells, a Ustrium modifying the pedicle 

 slit, and witltout piseiidodeltidia and false cardinal areas. Dorsal protegulum 

 usually subcentral. Ordovician to Recent. 



Family 1. Trematidae Scliuchert. 



Primitive Discinacea, in which the posterior margin of the ventral valve has a 

 triangular pedicle notch throughout life. A Ustrium is usually present. Ordovician 

 to Coal Measures. 



Trematis Sharpe (Orhicella d'Orb. 1847, non Dana 1846). Ventral valve 

 unevenly convex, more or less depressed over the posterior region. Pedicle 

 fissure large, extending 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. 



Eunoa Clarke. Very large, depressed, thin shells, with fine concentric 

 lines. Pedicle notch very large. Ordovician ; New York. 



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. 



Lingulodiscina Whitfield {Oehlertella Hall and Clarke). Much like Schizo- 

 crania, but the ventral valve has concentric growth lines, and no radiating 

 striae. Ventral pedicle area greatly elevated and transected by a narrow 

 open fissure. Devonian to Lower Carboniferous ; North America. 



Schizoholus Ulrich. Devonian ; North America. (?) Monobolina Salter. 

 May be an Obolid. 



Family 2. Discinidae Gray. 



Derived Discinacea with an open pedicle notch, in early life, in the posterior 

 margin of the ventral valve, which is closed posteriorly during neanic growth, leaving 

 a more or less long, narrotv slit, partially closed by the listrium. Ordovician to 

 Recent. 



Orbiculoidea d'Orbigny (Fig. 556). Shells inequivalved, sub-circular or 

 sub-elliptical in outline. Apices eccentric. Ventral valve deperssed, convex 



