CLASS I PELECYPODA 461 



Siiperfamily 8. ANOMIACEA Herrmannsen. 



SJiell monomy avian, not alate ; edentulous or isodont, usually sessile; shell substance 

 nacreous, tuhuUferous, with traces of a prismatic layer; area obscure, usually small, 

 amphidetic ; ligament obscure, with an alivincular internal rcsilium; (jills filihranchiate, 

 mantle lobes free ; foot small, grooved, digitiform ; dioecious; marine. 



Family 37. Anomiidae Gray. 



^liell variable, irregular and inequivalve lohen sessile, byssiferous lohen young ; in 

 most genera the byssus becomes modified to a calcified or horny plug passing through 

 a foramen in the right valve, and fastened to other objects, a condition which may be 

 permanent or transient-; area small, amphidetic; ligament amphidetic, more or less 

 internal, supplemented by an internal resilium, for which the crura serve as chondrophores, 

 ali- or multivincular ; hinge usually edentulous, rarely rugose, with amorphous inter- 

 locking rugosities ; posterior adductor small, sub-central, in the sessile forms reinforced by 

 the pedo-byssal muscles, which are modified for service as addtictors. (?) Devonian. Jura 

 to Recent. 



Anomia Miill. Shell thin, sessile by the calcified byssus 

 or perforation in the right valve, conforming to the subjacent 

 more convex, with four muscular scars on 

 a central area ; a chondroishore in the 

 lower valve. Jm-a to Recent. 



(?) Limanomia Boych. Devonian. 

 Hypotrema d'Orb. Jura and Cretaceous. 

 Placunanomia Brod. Miocene to Recent. 



Garolia Cantraine. (Fig. 725). Shell 

 orbicular, compressed, radially striated ; 

 right valve with a byssal foramen nearly 

 closed in the adult ; resilium much as in 

 Anomia ; adductor scar single. Eocene ; 

 Egypt. 



passing 

 sur&ce 



throiigh a sinus 

 ; the left valve 



Subgenus : JVakuUina Dall. Smooth ; 

 byssal foramen obsolete ; the resilium received 

 on diverging crura on the upper valve. Oligo- 

 cene ; Florida. 



Brug. : 



Fig. 725. 



CaroUa placunoides Caiitr. Eocene ; Wadi el Till, near 

 Cairo, Egypt. Interior of both valves, 2/3. 



Placenta Retzius [Placuna 

 Placunema Stol. ; Pseudoplacuna Mayer) 

 Shell free, orbicular, thin, very compressed ; 

 the resilium with long, unequal crura. 

 Tertiary and Recent. 



Ephippium Bolten. Like Placenta, 

 but the shell radially waved ; yoiuig with a small byssal perforation, which becomes 

 closed and obsolete in the adult. Tertiary and Recent. 



Placunopsis Morr. and Lye. Shell rounded, imperfoi-ate, free, or sessile. Jura. 



Hemiplicatala Desh. {Semiplicatula Fisch.) ; Saintia Rainc. Eocene. Paranomia 

 Conrad. Ripley Group. Monia Gray. Miocene to Recent ; California. 



Section E. Dysodonta Neumayr (emend.). 



Superfamily 9. MYTILACEA Fdrussac. 



Shell anisomyarian, usually equivalve, not alate or notched for a byssus, edentulous or 

 dysodont ; shell substance sub-nacreous, rarely more or less prismatic, with a conspicuous 



