378 CAMBKIAN BRACHIOPODA. 



while in all other Oboliis species they converge. As the species mentioned differ from each other considerably in other 

 respects, it seemed to me necessary to assign them to special subgenera, Thysanotos and Acritis. The remaining species 

 also fall into two groups differing from each other in many ways, one of them, Schmidtia, approaching more the typical 

 Obolus, the other, Leptevibolon, being closer to Lingula. 



The delimitation of the species presents similar difficulties to those in the allied genus XAngula. The differences 

 are manifest, but are difficult to express in words. This is true especially of some species of the subgenera Euoholus 

 and Schmidtia. 



The introduction of the subgenus Euoholus for the radially striated OboH does not appear 

 to be necessary for a proper classification of the genus. If the radially striated species are 

 considered to be the typical form of Oholus, then those difl^ering from it to a sufficient degree may 

 be referred to a subgenus. The genus Lingulella has only radially striated forms included 

 within it, and this in part includes species that otherwise would be referred to Euoholus. To 

 tliis extent Euoholus is a synonym of Lingulella. 



I have endeavored to identify in material from the typical localities the varieties of 

 Oholus and its subgenera described by Mickwitz, but find it practically impossible in nearly 

 all cases. They may be found of value to the special student, but they are not of serAdce in 

 studies of the Cambrian fauna. Such minute discriminations would lead to an endless amount 

 of detailed description if followed in the study of the faunas as they occur in the collections 

 before me. 



Fordinia Walcott [1908d, p. 64] includes specimens that have a Lingulella-like outline, with 

 the development of a tendency to form a platform or thickening in the valves. Type: Oholus 

 (Fordinia) perfectus Walcott. 



Leptemholon Mickwitz [1896, p. 199] is referred to in this monograph as a subgenus of 

 Lingulella. 



Lingulepis Hall [1863, p. 129] is referred to in this monograph as a subgenus of Lingulella. 



Linguloholus Matthew [1895b, p. 260] is a Lingulella-like form, with a thick, strongly arched 

 shell and surface marked by subimbricated wavy concentric lines. Type: Lingulella? affinis 

 Billings (PL XVI, figs. 1, la-e). 



Mickwitzella Walcott [1908d, p. 70] is an Oholus with strong, tmiformly curved concentric 

 strise, with the lamellse of growth fringed along their anterior (external) edges. Type: Oholus 

 siluricus Eichwakl (PI. XV, figs. 1, la-c). 



Monoholina Salter [1866b, p. 334] has been referred to as a subgenus of Oholus by Matthew 

 [1902b, p. 98]. It differs from Oholus in having a platform in the ventral valve of the same type 

 as that of Elkania (PI. LI, figs. 1, la, and 4b) and is clearly not descendant from Oholus or 

 generically connected with it. Salter [1866b, p. 334] placed Monoholina as a section of the 

 genus Oholella Billmgs. 



Palseoholus Matthew [1899, p. 201] shows concentric lines which are elevated and slightly 

 irregular. The visceral area (splanchnocoele) is large ajid elongate, and the main vascular canals 

 are close to the visceral area (PL XXXII, figs. 5, 5a-g). The surface of Palseoholus is of the same 

 type as that of Acritis, but the visceral cavity and vascular canals differ materially in the two 

 forms. Type: Palseoholus hretonensis Matthew. 



Schmidtia Volborth [1869, p. 208] is a Lingulella-like shell without radial striation. All the 

 species are small and as the concentric striation is very fine the shell surface is nearly smooth. 

 Type: Schmidtia celata Volborth (PL XIV, figs. 1, la-c). 



TTestoma Walcott [1901, p. 691] is a Lingulella-like form distinguished by peculiar, transverse, 

 semi-imbricating, "ripple-embossed" lines that cross both the concentric and radiating striae. 

 Type: Lingula aurora Hall (PL XLVI). 



Stratigraphic range of Oholus and subgenera. — Oholus has a stratigraphic distribution rang- 

 ing from the limits of the Olenellus fauna, through the Middle Cambrian or Olenoidesi&MaB, and the 

 Upper Cambrian or DilieTlocephalus {Olenus] fauna, into the base of the Ordovician fauna. Oholus 

 proper has not been found below the upper portion of the Lower Cambrian. The youngest 

 species of the genus are the lai'gest, and also have certain strongly marked characteristics which 



