ORDER i PRIONODESMACEA 861 



Order 1. PRIONODESMACBA. Ball. 



I. (PALAEOCONCIIA, ]>.j>. Xeumayr). 1 

 Family 1. Solemyacidae. 



Shell Soleniform, equivalve, low -beaked, edentulous, gapiini, >///// the anterior end 

 longer and the epidermis conspicuous, exceeding the valves ; area obscure or none ; ligament 

 amphidetic, parivinevliar t /"<//////// internal posteriorly; mantle lobes united ventrally, 

 attached in front to the epidermis and valves by a broad surface, leaving no distinct pallial 

 line; a single posterior siphonal and anterior pedal foramen in the mantle ; adductors 

 sub-equal, with a thickened ray in front of the posterior scar : uniuml dioecious, marine, 

 burrowing. Silurian to Recent. 



Solemya, Lam. Carboniferous to Recent, rare in all horizons. 1 0rthodesma, 

 Hall. Silurian. Janeia, King, shorter and less inequivalve, may include most of the 

 Palaeozoic species hitherto referred to Solemya. Clinopistha, Meek and Worthen, 

 from the Carboniferous, is also allied, and Dystactella, Hall, is united with it by Zittel. 

 Phthonia, Hall, from the Devonian, is placed here by Ulrich. 



Family 2. Solenopsidae. Neumayr. 



Shell thin, elongate, equivalve, with very anterior beaks ; the hinge edentulous, ligament 

 parivincular, external; pallial line not sinuated ; a ridge or groove radiating from the 

 beak to the lower posterior angle of the valves. Marine. Silurian to Trias. 



Sanguinolites, M'Coy. Elongate, obliquely truncate behind ; beaks low, sculpture 

 of concentric or broken lines, anterior 

 adductor scar buttressed by a ridge. 

 Carboniferous. 



Promacrus and Prothyris, Meek. 

 Carboniferous. FIG. coo. 



Arcomyopsis, Sandb. Somewhat Solenopris pelagico, Goldf. Devonian ; Eifel District, 

 curved, with prominent beaks ; obliquely 



truncate behind ; posterior area radially, the rest of the surface concentrically 

 sculptured. Devonian. 



Orthonota, Conrad ; Orthodesma, Hall. Silurian. 



Solenopsis, M'Coy (Fig. 600). Very long, scabbard-shaped, smooth ; anteriorly 

 short and rounded, gaping behind. Devonian to Trias. 



Family 3. Vlastidae. Neumayr. 



Shell thin, very inequivalve, beaks elevated, hinge line edentulous, arched, meeting at an 

 obtuse angle beneath the beaks, leaving a dorsal opening ; surface smooth or concentrically 

 striate. 



The two genera Vlasta and Dux (Vevoda), Barrande, from the Silurian of Bohemia 

 (Etage E 2), constitute this family. 



Family 4. Grammy siidae. Fischer. 



Shell thin, equivalve, oval, or elongate, with the beaks sub-central to anterior; hinge 

 edentulous, sometimes thickened. Ligament parivincular, external; pallial line not 

 sinuate; surface smooth or concentrically sculptured. Silurian to Carboniferous. 



1 [The terms Palaeoconcha, Taxodonta, Schizodonta, etc., preceded by Roman numerals, un> 

 retained here merely as convenient descriptive appellations, and are in nowise to be regarded as 

 possessing systematic values.] 



