172 



PALEOZOIC TIME. 



Trilobites of the genus Paradoxides (Fig. 251), none of which are 

 known afterward. 



4. Ortkis Family. — In the genus Orthis (Figs. 235-237) the species are usually rather 

 thin ; often orbicular, at times a little wider than high ; both valves in general nearly 

 equally convex ; the hinge-line usually not long, with a small cardinal area; a few 

 species resemble a narrow Spirifer, and have a median fold and long hinge-line. Or- 



Figs. 226-237. 



Fig. 226, Rhynchonella psitfcacea, showing the spiral arms of the animal ; 227, id dorsal valve ; 

 228, id. ventral ; 229, Strophomena planumbona; 239, id. dorsal valve; 231, id. ventral; 232, 

 Leptsena transversalis ; 233, id. dorsal valve; 234, id. ventral; 235, Orthis ptriatula ; 236, id. 

 dorsal valve ; 237, id. ventral. 



thisina has the hinge-area very large and reversed-triangular, with a convex deltidium, 

 and the shell subquadrate. Strophomena contains thin D-shaped species (Figs. 229- 

 231), with a straight hinge-line about as long as the width of the shell, a very nar- 

 row hinge- area, the dorsal valve often very concave, with the ventral bending to cor- 

 respond, and the four adductor muscular impressions in the same transverse line. 

 Leptcena is similar (Figs. 232-234), but has the four muscular impressions of different 

 character, as seen in Fig. 233, while in Strophomena they are as in Fig. 230. 



5. Productus Family. — In the genus Productus (Figs. 238, 239) the beak is very full, 

 hinge-line usually a little shorter than the width of shell ; no true hinge-area, and no 

 beak -aperture; the smaller valve concave ; the surface of the shell spinous, the spines 

 tubular; spiral arms present, but without calcareous supports. The margin of the 

 shell is prolonged downward, often to a great length, and sometimes closes around 

 into a tube Chonetes (Fig. 240) has a straight hinge-line, commonly as long as the 

 width of the shell, the form rather thin, with the beaks not full and prominent, 

 resembling Leptaena; smaller valve concave; hinge-edge of larger valve furnished 

 with a few spines. Strophalosia is much like Productus in form and spines, but 



