376 



MOLLUSCOIDEA 



PHYLÜM V 



Schizambon Walcott (Schizamhonia Q^hlert). Small spiniferous Siphono- 

 tretids with much of the pedicle sheath open as a cleft on the outside. 

 Upper Cambrian and Ordovician ; America and Russia. 



Trematobolus Matthew {ProtosipJion Matthew). With rudimentary articu- 

 lation and lamellose exterior. Lower and Middle Cambrian ; California and 

 New Brunswick. 



Yorkia Walcott. The oldest form of the family. Concentrically striated 

 exterior. Lower Cambrian of eastern North America. Dearhornia Walcott. 

 Middle Cambrian ; Montana. 



Keyserlingia Pander. Thick-shelled form with decided muscular impres- 

 sions and median septa in both valves. Lower Ordovician ; Esthonia. 



Superfamily 2. ACROTRETACEA Schuchert. 



Progressive Neotremata with corneous or calcareo-corneous shells that are more or 

 less circular in outline and from highly conical to depressed in form. The pedicle 

 opening is a simple, circular, more or less conspicuous Perforation through the apex 

 of the ventral valve, and is situated posterior to the protegulum. A listrium is not 

 developed. A false cardinal area is often present. Dorsal protegulum 7tiarginal. 

 Cambrian to Silurian. 



Family 1. Acrotretidae Schuchert. 



Same characters as superfamily. The large and depressed forms are of the 

 suhfamily Acrothelinae, white the small forms with more or less high ventral valves 

 are of the Acrotretinae. Cambrian to Silurian. 



Suhfamily A. Acrothelinae Walcott and Schuchert. 



Acrothele Linnarsson (Fig. 554). 

 cl 



Cambrian ; North America and Europe. 

 Subgenus : Redlichella Walcott. 

 > Middle Cambrian ; Sweden. 



Discinolepis Waagen. Middle 

 Cambrian ; India. 



Fig. 554. 



Acrothele {Redlichella) granulata (Linnarsson). Middle Cam- 

 brian ; Sweden. A, Ventral valve with its minute pedicle foramen. 

 B, Dorsal exterior. C, Acrothele matthewi (Hartt), showing vas- 

 cular Sinus (vs). Middle Cambrian ; New Brunswick (after 

 Walcott). 



A 



Suhfamily B. Acrotretinae 

 Matthew. 



B 



V5. 



3 ^•f\^§ 



A false cardinal area often 

 present. 



Acrotreta Kutorga (Linnars- 

 sonia C. D. Walcott) (Fig. 555). 

 Widely distributed throughout 

 the Cambrian and rarely in 

 the Ordovician of Europe and 

 North America. 



Acrothyra Matthew. Ven- 

 tral valve often excessively 

 high and oblique or elongate in outline. Middle Cambrian ; New Brunswick. 

 Conotreta Walcott. Highly conical shells with the ventral interior marked 

 by a number of radiating ridges. Ordovician and Silurian ; North America. 



Fig. 555. 



A-C, Acrotreta schmalensii Walcott. Middle Cambrian ; 

 Sweden. A, Three views of the ventral exterior, B, Ventral 

 interior with vascular sinus (vs). C, Dorsal interior. D, E, 

 Acrotreta subconica Kutorga. Ordovician ; Esthonia. Cardinal 

 view and dorsal valve (after Walcott). 



