CAMI5RIAX BRACniOPODA WALCOTT 59 



MICROMITRA (PATERINA) WAPTA, new species 



Plate 7, Figure 6 



Shell large and thick for a species of- this genus. A'entral valve 

 depressed conical, with the apex above a narrow false area that is 

 outlined by the abrupt curvature of the shell. As the shells usually 

 occur compressed in the siliceous shale, the false area is concealed 

 and the posterior slopes from the apex form a blunt angle at the 

 apex. Dorsal valve transverse, moderately convex, with the pos- 

 terior margin nearly straight and a little shorter than the greatest 

 width of the valve ; beak small, marginal ; cardinal slope and false 

 area unknown. 



Surface marked by concentric, slightly irregular, rounded lines 

 and ridges of growth that are grouped in bands of varying width ; a 

 few radiating striae or lines occur on the central portions of one 

 ventral valve ; with a lens magnifying 20 diameters, an occasional 

 roughness can be seen in reflected light on the surface of some of 

 the concentric ridges. 



Observations. — This is one of the largest species of the genus. 

 One ventral valve has a length and breadth of 14 mm. and several 

 are 9 to 11 mm. in diameter. It compares in size with Micromitra 

 (Iphidella) nyssa (see p. 57), from the same geological horizon in 

 Montana, but the latter has a reticulate exterior surface of the M. 

 (I.) pannula (White) [1874, p. 6] type. It was at first thought that 

 this species might be the old shells of Acrothcle coUcni, new species, 

 but a careful comparison with the younger stages of growth of MAP.) 

 zcapta shows that the latter has only indefinite traces of the highly 

 ornate surface of Acrothcle collcni, and that the apex of the ventral 

 valve of M. (P.) zi'apta is imperforate and over the posterior margin 

 and not on the general surface of the valve in advance of the margin, 

 as in Acrothcle colleni. The two species wxre found associated on 

 Mount Bosworth. M. (P.) zvapta is of the same type as M. (P.) 

 labradorica (Billings) [1861&, p. 6], il/. (P.) prospccteusis (Wal- 

 cott) [1884, p. 19], and il/. (P.) stissingoisis (Dwight) [1889, 

 p. 145]. It differs from all in having more irregular, less definite 

 threadlike concentric lines, and in the manner in which the striae are 

 assembled in ridges. 



FoRMATiox and LOCALITY. — Lower Cambrian : Drift block of 

 siliceous shale supposed to have come from the ^Nlt. ^Mlyte forma- 

 tion [Walcott, 19080^ p. 4], south slope of Mount Bosworth, on the 

 Continental Divide, one mile (1.61 km.) west of Stephen, on the 

 Canadian Pacific Railway, British Columbia, Canada. 



