PATERINID^. 



355 



Figure 26.— Micromitra (Paterina) stuarti Walcott. A, A', A", Top, side, and back views of ventral valve, 

 the type specimen (U. S. Nat. Mus. Cat. No. 51485a). B, B', Top and side of dorsal valve (U. S. Nat. 

 Mus. Cat. No. 51485b). 



The specimens represented are from Locality 54n, Middle Cambrian limestones in Blacksmith Fork Can- 

 yon, east of Hyrum, Cache County, Utah. Figures 26A and 26A" are copied from Walcott [1908d, PI. VII, 

 figs. 8 and 8a]. 



Dorsal valve rather strongly convex for a species of this genus; the highest pan is at 

 about the center of the shell, from whence the slope is very sUght to the beak, and rather rapid 

 to the front margin. 

 Beak marginal above 

 a low, broad arching 

 of the posterior mar- 

 gin of the shell; area 

 shown only by a yerj 

 narrow margin where 

 the shell bends toward 

 the median line; no 

 trace of a pseudodel- 

 tidium has been ob- 

 served. 



Surface marked 

 by narrow, rounded, 

 concentric threadlike strise or ridges with short striae between them. Shell substance corneous. 



The average size of adult shells is 8 mm. long by about the same width. 



Observations. — Tliis is one of the larger species of the genus. Micromitra (Paterina) superha 

 (Walcott) occurs 16 feet (4.5 m.) below and Micromitra {Ifliidella) pannula (White) 70 feet 

 (21.3 m.) below in the same section. 



This fine shell has a short pseudodeltidium much like that of M. (P.) logani (Walcott), 

 but it differs in form and greater size; the same is true of M. (P.) crenistria (Walcott). It 

 may be closely related to M. (P.) labradorica utahensis (Walcott), but the specimens of the 

 latter are too imperfect for close comparison of form. 



The specific name is given for my son, Benjamin Stuart Walcott, who assisted me in col- 

 lecting the specimens during the summer of 1906. 



Formation and locality. — Middle Cambrian: (54n) About 550 feet (167.6 m.) above the Brigham quartzite 

 and 3,640 feet (1,109.5 m.) below the Upper Cambrian, in the limestones forming 2e of the Ute limestone [Walcott, 

 1908f, p. 197], in Blacksmith Fork Canyon, about 10 miles (16.1 km.) east of Hyrum, Cache County, Utah. 



Micromitra (Pateeina) supeeba (Walcott). 



1/ Text figures 27A-B; Plate II, figures 7, 7a-f. 



Iphidea cnf.? ornatella Hall and Clarke, 1892, Nat. Hist. New York, Paleontology, vol. 8, pt. 1, PI. IV, figs. 6 and 7. 

 (No text reference.) 



Iphidea superha Walcott, 1897, Proc. U. S. Nat. Mus., vol. 19, p. 711, PI. LIX, figs. 1, la-c. (Described. The speci- 

 mens represented by figs. 1, la-c are redrawn in this monograph, PI. II, figs. 7c, 7, 7a, and 7b, respectively.) 



Ventral valve subconical, with a minute beak incurving over the pseudodeltidium. Car- 

 dinal slope slightly flattened so as merely to indicate an imperfectly defined rather narrow 



false area. In some specimens 

 the curvature of the shell is 

 practically continuous to the 

 base of the pseudodeltidium. 

 Pseudodeltidium broad, con- 

 vex, with its lower margin 

 broadly arched, so as to leave 

 a considerable space between 

 it and the line of the general 

 plane of the shell. 



Dorsal valve slightly con- 

 vex; most elevated a little in 

 front of the small beak which projects a little over the broad open space beneath it. No traces 

 of a false area or pseudodeltidium have been observed. 



FiGXTEE 27 .— Micromitra (Paterina) supcrba (Walcott). A, A', Top and back views of ventral 

 valve from Locality 84n, Middle Cambrian limestones in Blacksmith Fork Canyon, Cache 

 County, Utah (U. S. Nat. Mus. Cat. No. 51486). B, Side view of ventral valve showing 

 pseudodeltidium, from Locality 5iy, at the same locality but from a slightly lower horizon 

 than that of the specimen represented by figures 27A-A' (U. S. Nat. Mus. Cat. No. 51491). 



