602 HISTORY OF SYSTEMS. 



23. Trigonacea. Shell regular ; hinge teeth lamel- 



lar, grooved transversely. Trigonia. 



24. Arcacea. Shell close, or gaping inferior!? ; 



hinge multidenticulate, straight or curved, or 

 angular. Nucula, Pectunculus, Area, Cuculaea. 



25. Mytilacea. Shell regular, fixed by a byssus ; 



hinge simple, edentulous or subdentate ; liga- 

 ment marginal subinternal. Mytilus, Pinna, 

 Avicula. 



* * Shell irregular. 



26. EtheriDjE. Shell irregular, adherent; hinge 



edentulous ; ligament in a gutter of the beaks. 

 Etheria. 



27. Rudistes. Shell irregular, adherent ; upper 



valve operculiform, two prominent muscular 

 impressions in the upper valve ; ligament in- 

 ternal. Spherulites, Hippurites, Caprina. 



MONOMYARIA. 

 Order I. 



28. Malleacea. Shell free, subinequivalve, with a 



simple or compound ligament, subinternal in 

 one or several grooves or crenulations. Cre- 

 natula, Perna, Inoceramus, Catillus, Gervilia, 

 Malleus, Vulsella. 



29. Mulleria. Shell adherent; hinge edentulous ; 



ligament external. Mulleria. 



30. Pectinides. Shell free or adherent, auricled; 



hinge straight ; ligament internal or semi-in- 

 ternal. Lima, Plagiostoma, Pecten, Hinnites, 

 Plicatula, Spondylus. 



Order II. 



31. Ostracea. Shell irregular, foliated, free or 



adherent ; ligament internal or semi-internal. 

 Placuna, Anomia, Ostrea.f 



Class— TUNIC AT A. 



I can scarcely consider the alterations which have been 

 made in the classification of Savigny as improvements upon 

 it. The rank of the Tunicata must be granted to be the 



t Trait. Element, i. 212. This classification was proposed in 1830. See 

 also Deshayes' article " Conchifera" in Cyclop. Anat. and Physiology, i. 695. 



