PLATE XCVI. 
e. Crura. ps. Pseudospondylium. 
j. Cardinal process. s. Median septum, 
t. Teeth. 
EoorTHIS DESMOPLEURA (Meek) (p. 777). 
Fieures 1; la, la’; 1b; and 1c. Exterior and side views of four ventral valves, illustrating variation in form and 
surface characters. Locality 187, Lower Ordovician red siliceous limestone 105-122 feet above the 
pre-Cambrian, 2 miles (3.2 km.) below Manitou Park Hotel, El Paso County, Colorado. U.S. Nat. 
Mus. Cat. Nos. 52317a-d, respectively. 
Idand le. Cast of ventral valve associated with the specimen represented by figure 1, with side outline, show- 
ing area and cast of umbonal cavity. U.S. Nat. Mus. Cat. No. 52317e. 
lf and 1g. Dorsal valves associated with the specimen represented by figure 1. U.S. Nat. Mus. Cat. Nos. 
52317f and 52317g, respectively. 
Ih. Silicified shell showing character of radiating costee and strie. This is the type specimen, from Locality 
360f, Lower Ordovician siliceous limestone at Glen Eyre, Camp Creek Canyon, near Manitou, El 
Paso County, Colorado. U.S. Nat. Mus. Cat. No. 7859a. 
liand 1j. Top and side views of ventral valves from Locality 14k, Upper Cambrian limestone on Wolf Creek, 
15 miles (24.2 km.) west-southwest of Sheridan, Sheridan County, Wyoming. U.S. Nat. Mus. Cat. 
Nos. 52320a and 52320b, respectively. 
1k and 11. Exterior of dorsal valves associated with the dorsal valves represented by figures liand 1j. U.S. 
: Nat. Mus. Cat. Nos. 52320c and 52320d, respectively. 
Im and In. Exterior of small ventral valves. U.S. Nat. Mus. Cat. Nos. 52319a and 52319b, respectively. 
lo. Exterior of a ventral valve. U.S. Nat. Mus. Cat. No. 52319¢c. 
1p. Side view of a young, convex shell. U.S. Nat. Mus. Cat. No. 52319d. 
1q. Posterior portion of the interior of an abraded ventral valve. U.S. Nat. Mus. Cat. No. 52319e. 
lr. Interior of an abraded dorsal valve. U.S. Nat. Mus. Cat. No. 523109f. 
The specimens represented by figures lm-r are from Locality 168, Middle Cambrian limestones on Tepee Creek, 
near the road from Sheridan to Dome Lake, Bighorn Mountains, Wyoming. : 
EoorTHIS DESMOPLEURA NYMPHA (Walcott) (p. 778). : 
_Ficure 2. Exterior and side views of a ventral valve, the type specimen, from Locality 168, Middle Cambrian lime- 
stones on Tepee Creek, near the road from Sheridan to Dome Lake, Bighorn Mountains, Wyoming. 
U.S. Nat. Mus. Cat. No. 52333a. : 
PLECTORTHIS WHITFIELDI (N. H. Winchell) (p. 308). 
Ficure 3. Interior of a ventral valve, showing the relatively high area and strong teeth, the outline of the pseudo- 
spondylium, and the low ridges extending forward and outward from its anterior margin. Ordovician, 
Spring Valley, Minnesota. Copied from Hall and Clarke [1892c, Pl. V, fig. 26]. 
PLECTORTHIS PLICATELLA (Hall) (p. 772). 
[Text fig. 12, p. 299.] 
Figure 4. Posterior portion of the interior of a ventral valve, showing the broad delthyrium, teeth, and the outline 
of the pseudospondylium. 
da. The area of the dorsal valve, showing the triangular delthyrium, cardinal process, crural plates, and low 
narrow median ridge. 
The specimens represented are all from the Ordovician at Covington, Kentucky. The figures are copied from Hall 
and Clarke [1892c, Pl. V, figs. 19 and 20]. 
320 
