336 CAMBRIAN BEACHIOPODA. 



Micromitra sculptilis (Meek), the type of group A (Micromitra Meek), is therefore inter- 

 mediate in surface characters between the concentrically striated forms of group B {Paterina 

 Beecher) and the network ornamentation of group O (IpMdella Walcott), though some exam- 

 ples of the latter group have both the A and types of surface on the same shell. The species 

 with the intermediate type of surface may be grouped under the genus Micromitra, as follows: 



Micromitra alabamaensis (Walcott) Middle Cambrian. 



haydeni Walcott Do. 



nisus (Walcott) Do. 



pealei (Walcott) Do. 



. pusilla (Linnarsson) Do. 



Bcotica Walcott Lower Cambrian. 



sculptilis (Meek) Middle and Upper Cambrian. 



sculptilis endlichi Walcott Middle Cambrian. 



zenobia Walcott Do. 



Micromitra {Paterina) lahradoricaswantonensis (Walcott) and M. (P.) iella (Billings) represent 

 the second t3^pe (B), whose surface is formed of simple concentric striae and lines of growth. On 

 the basis of surface characters this group is given a subgeneric value, as suggested by Schuchert 

 [1905, p. 329], and the name Paterina of Beecher [1891, p. 345] is adopted."^ The group includes: 



Micromitra (Paterina) bella (Billings) Lower Cambrian. 



crenistria (Walcott) Middle Cambrian. 



etheridgei (Tate) Do. 



labradorica (Billings) Lower Cambrian. 



orientalis (Walcott) Do. 



swantonensis (Walcott) Do. 



utahensis (Walcott) Middle Cambrian. 



var. undt Lower Cambrian. 



logani (Walcott) Middle Cambrian. 



major (Walcott) Lower Cambrian. 



phillipsi (Holl) Upper Cambrian. 



prospectensis (Walcott) Lower Cambrian. 



stissingensis (Dwight) Middle Cambrian. 



stissingensis ora Walcott Do. 



stuarti Walcott Do. 



superba (Walcott) Do. 



undosa (Moberg) Lower Cambrian. 



wapta Walcott Do. 



williardi Walcott Do. 



Micromitra (IpJiidella) ornatella (Linnarsson) and M. (7.) pannula (White) represent the 

 third type (C), whose surface is formed by the union of the crenulated striae so as to form a 

 fine network of raised obliquely arranged lines that divide the surface into minute pits that 

 give the impression of finely woven cloth. For the species with this ornamented surface 

 IpMdella Walcott [1905a, p. 304] is used in a subgeneric sense. This includes: 



Micromitra (Iphidella) louise Walcott Lower Cambrian. 



nyssa Walcott Middle Cambrian. 



ornatella (Linnarsson) Do. 



pannula (White) Lower and Middle Cambrian. 



maladensis (Walcott) Middle Cambrian. 



ophirensis (Walcott) Do. 



StratigrapJiic distrilution. — The genus ranges from low down in the Lower Cambrian 

 through the Mddle Cambrian, and there are two species in the LTpper Cambrian. 



Of the Lower Cambrian species, M. (7.) pannula (White) passes up into the central portion 

 of the Middle Cambrian in the Marjum limestone of the House Range section, L^tah. Micro- 

 mitra sculptilis (Meek) occurs abimdantly in the central portion of the Middle Cambrian and 

 has also been found in the Upper Cambrian Dunderberg shale of the Eureka district, Nevada. 



Micromitra {Iphidella) pannula ophirensis (Walcott) has a range of 2,370 feet (722.4 m.) 

 in the House Range section of Utah, occurring in the Middle Cambrian at horizons 220, 2,075, 

 and 2,590 feet (67, 632.5, and 789. 4 m.) above the quartzitic sandstones of the Lower Cambrian. 



n It is therefore impossible to use the type of the preoccupied genus Iphidea, Micromitra {Paterina) bella, as the type of this subgenus, as 

 Beecher founded it on " Obolus labradorieus." (See p. 344.) 



