66o 



MOLLUSCOIDEA. 



ornamented with striae, or furnished with well-marked longitudinal 

 ribs. The species of the genus Orthis begin in the Cambrian, and 

 abound in the Ordovician, Silurian, Devonian, and Carboniferous 

 periods, especially in the first two of these ; but the genus is not 

 known to have survived the Carboniferous period. This genus is 

 one of the most important and widely distributed groups of Palaeo- 

 zoic Brachiopods, and the species belonging to it may generally be 

 distinguished from the closely related Strophomence, by the fact that 

 the shell is seldom flat, one valve being usually much more convex 

 than the other ; while the general form is compact and not ex- 

 tended, and the hinge-line is often shorter than the greatest width 



Fig. 514. — <2, Dorsal aspect of Strophomena alternata, from the Ordovician of North America ; 

 b, Ventral aspect of Strophomena jilitexta, Ordovician, North America ; c, Orthis testudin- 

 aria, Ordovician; d d, Orthis plicatella, Ordovician; e e' e" , Leptcena sericea, Ordovician. 

 (After Meek, Hall, and the Author.) 



of the shell. The muscular scars are quadrate, and not extended 

 either vertically or laterally. Lastly, the " cardinal process " — that 

 is, the projection of the dorsal valve, to which the " divaricator 

 muscles" are attached — is undivided and linear. Though the 

 species of Orthis most nearly resemble certain forms of Stropho- 

 mena, and are very liable to be confounded with these, one or 

 two species (such as Orthis biforata — 5 1 3, a) closely simulate the 

 genus Spirifera in general form. 



In the sub-genus Ptaty strop hia, of which Orthis biforata of the Ordo- 

 vician rocks (fig. 513, a)'\% a characteristic form, the shell is generally 

 transversely elongated, with a long hinge-line, both valves convex, and 

 the ventral valve with a deep median sinus. A hinge-area and triangu- 

 lar deltidial fissure are present in both valves. The valves are radially 

 plaited, and the beaks are prominent and incurved. The sub-genus 

 ranges from the Ordovician to the Carboniferous. 



The genus Skenidium (fig. 515, D — G) is in many respects allied to 

 Orthis, but the ventral valve is much elevated, with a high triangular 



