58 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 53 



Subgenus PATERINA Beecher [1891, p. 345] 

 MICROMITRA (PATERINA) STUARTI, new species 



Plate 7, Figures 8 and 8(? 



Ventral valve subconical, with a minute beak arching slightly over 

 a short pseudodeltidium. Cardinal slope with a rounded angle that 

 extends from the beak to the postero-lateral margin and defines a 

 very narrow, flattened area on each side of a high, triangular fissure 

 that is covered for a short distance at the top by a very short, arched 

 pseudodeltidium. 



Dorsal valve rather strongly convex for a species of this genus ; 

 the highest part is at about the center of the shell, from where the 

 slope is very slight to the beak and rather rapid to the front margin. 

 Beak marginal above a low, broad arching of the posterior margin of 

 the shell ; area shown only by a very narrow margin where the shell 

 bends toward the median line ; no trace of a pseudodeltidium has 

 been observed. 



Surface marked by narrow, rounded, concentric thread-like striae 

 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. — This is one of the larger species of the genus ; it 

 occurs quite abundantly in a compact, bluish-gray limestone in the 

 lower portion of the Middle Cambrian terrane. Microuiitra 

 (Paterina) superba (Walcott) [1897^, p. 711] occurs 16 feet (4.8 m.) 

 below and M. (Iphidella) panmda (White) [1874, p. 6] 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) [1897, P- 7ii]» but it dififers in form and 

 greater size; the same is true of M. (P.) crcnistria (Walcott) 

 [1897, p. 713]. It may be closely related to M. (P.) lahradorica 

 utahensis (Walcott) [1905, p. 306], but the specimens of the latter 

 are too imperfect for close comparison of form. 



The specific name is given for my son Benjamin Stuart, who 

 assisted me in collecting the specimens during the summer of 1906. 



Formation and Locality. — Middle Cambrian : Limestones of 

 the Ute formation [Walcott, 1908a, p. 7], 185 feet (56.4 m.) above 

 the Cambrian quartzitic sandstone beds, in Blacksmith Fork Canyon, 

 about 8 miles (12.87 l^ni-) above its mouth and 15 miles (24.1^ km.) 

 east of Hyrum, Cache County, Utah. 



