524 TRANSACTIONS OF THE WAGNEK FREE 



to Ostrea less surprising, since Ostrea is now known, also, to be derived from 

 Pteriidce. 



Superfamily TRIGONIACEA. 



Shell equivalve, inequilateral, closed, dimyarian, not alate ; shell sub- 

 stance nacreous and prismatic; hinge-teeth few, subumbonal, typically schiz- 

 odont ; area obscure or none ; ligament parivincular, opisthodetic, external ; 

 ventricle bilobate, embracing the rectum, with anterior and posterior aortas; 

 gills filibranchiate, with direct and reflected laminae, the stems adherent behind 

 the foot ; mantle lobes usually free, but modified on the posterior edges to form 

 functional siphons though without conjunctive partitions ; pallial line usually 

 simple ; foot long, compressed, produced before and behind, capable of sub- 

 reptary expansion ; nonbyssiferous, though possessing an obsolete byssal 

 apparatus ; anal end of rectum adherent ; young without a distinct nepionic 

 stage ; dioecious ; station marine. 



Family LYRODESMID^. 



Shell with the hinge armature radiating tanlike from below the umbones, 

 teeth 5-9 ; the pallial line feebly sinuate or simple. 



Silurian. 

 Ex. Lyrodesma, ? Aclinodonla. 



Family TRIGONIIDyE. 



Shell with few hinge-teeth (f), the mantle lobes wholly free, but so ap- 

 plied to each other in life as to form functional siphons ; pallial line simple ; 

 palpi not appendiculate. 



Devonian to recent fauna. 

 Ex. Trigonia, Myophoria, Schizodus, ? Cythereodon. 



The characters of the soft parts in this family are enumerated under the 

 head of the superfamily. The posterior part of the foot in the recent species, 

 as I have personally observed, can be flattened so as to form a disk with crenu- 

 lated edges, resembling the reptary disk of Nitcula, but it is not, like the latter, 

 formed by a sulcation or splitting of the foot, and is used as a fulcrum, rather 

 than for crawling, as in some Macomas. It possesses a large blood sinus, as in 

 Cardhini, and, like the foot of Cardiuni, is specialized for use in leaping. The 

 prodissoconch in Trigonia pectinata is of the ordinary smooth, globular type. 



C. Isodonta. 

 Normal armature of the hinge obsolete, and its functions performed, if 

 at all, by modifications of the auricular crura, which form interlocking processes. 



Superfamily PECTINACEA. 



Shell usually inequivalve, flabelliform, more or less auriculate, and mono- 

 myarian ; shell structure subnacreous, corrugated and rarely prismatic, occasion- 



