376 MOLLUSCOIDEA phylum v 



ScMzamhon Walcott (Schizambonia (Ehlert). Small spiniferous Siphono- 

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

 Upper Cambrian and Ordovician ; America and Russia. 



Trematoholus Matthew {ProtosijjJwn Matthew). With rudimentar}'- 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. 



KeyserUngia 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 consjncuous 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 marginal, 

 Cambrian to Silurian. 



Family 1. Acrotretidae Scliiichert. 



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

 sid)family Acrotheliiiae, tuhile the small forms tuith more or less high ventral valves 

 are of the Acrotretinae. Cambrian to Silurian. 



Subfamily A. Acrothelinae Walcott and Sclincliert. 



Acrothele Linnarsson (Fig. 554). Cambrian ; North America and Europe. 



Subgenus : liedlichella Walcott. 



"^V '^--Jvs B 



S^ gl 



Middle Cambrian ; Sweden. 

 /(' -I'll IHscinolepis Waagen. Middle 



Cambrian : India. 



} 



'"■ ^^'*' Subfamily B. Acrotretinae 



Acrothele (RedlicheUa) grannhita (Linnarsson). Middle Cam- -.j- . .-i 



brian; Sweden. .1, Ventral valve witli its minute pedicle foramen. IViatuiew. 



B, Dorsal exterior. (.', Acivthele imUtlwivl (Hartt), showing vas- 



wiicott)""'' ^''^' ^''^'"'' ^^'"'''■'''" ' ^^""^ Brunswick (after J false Cardinal area often 



present. 



A B C D jL E Acrotreta Kutorga (Linnars- 



fTl vs ^^^ ''f%s ^ ?«ni s^'^^'^ C. D. Walcott) (Fig. 555). 



' ^ N^ ^ W ^ \ \ ' ^ Widely distributed throughout 



Fig. 555. "~^ the Cambrian and rarely in 



A-C, Acrotreta schnudcn.sii Walcott. Middle Cambrian ; the OrdoviciaU of Europe and 



Sweden. A, Three views of the ventral exterior. li, Ventral at ^-V, A " 



interior with vascular sinus (rs). C, Dorsal interior. D, E, JN Ortu America. 

 Acrotreta mhronica V.\\Un-'^a.. Ordovician; Esthonia. Cardinal /I pvnihin-n TV/To ffl-iot.r \T^-^ 



view and dorsal valve (after Walcott). ^ troiliyi a IViattne W. V eU- 



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. 



