ORDER I PRIONODESMACEA 361 
Order 1. PRIONODESMACEA. Dall. 
I, (PaLaroconcna, p.p. Neumayr).! 
Family 1. Solemyacidae. 
Shell Soleniform, equivalve, low-beaked, edentulous, gaping, with the anterior end 
longer and the epidermis conspicuous, exceeding the valves ; area obscure or none; ligament 
amphidetic, parivincular, becoming 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 ; animal dioecious, marine, 
burrowing. Silurian to Recent. 
Solemya, Lam. Carboniferous to Recent, rare in all horizons. % Orthodesma, 
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. 
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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. Fra. 600. 
Arcomyopsis, Sandb. Somewhat Solenopsis pelagica, 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. Vilastidae. 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.. Grammysiidae. 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, ete., preceded by Roman numerals, are 
retained here merely as convenient descriptive appellations, and are in nowise to be regarded as 
possessing systematic values. | 
