434 CAMBRIAN BRACHIOPODA. 



areas occupied by tlie muscle scars (PI. XVI, fig. 2j). This latter character is probably what 

 Mickwitz [1896, p. 69] describes as the longitudinal striation of the area in advance of the 

 visceral cavity in Oholus. The parietal scar is distinctly shown in advance of the visceral 

 area of the dorsal valve, where its course may be followed from the median line in front of 

 the anterior lateral muscle scars to the outward curve to the main vascular siauses, beyond 

 which it has not been observed. In none of the specimens of the Ventral valve can the exact 

 position of the parietal scar be determined. 



Observations. — The study of the material which I collected at the tj^pical locality in 1888 

 shows clearly that this species is very closely related to Oholus. It differs in having a smaller 

 area and somewhat m the form and convexity of the valves. There are no special characters 

 that do not apparently fall well within the limits of Oholus. There is certainly not more than 

 a subgeneric difference between the different forms of Oholus and Oholus (Linguloholus) spissus. 

 Billmgs [1872b, p. 467] figured a ventral and a dorsal valve, but furnished a very inade- 

 quate illustration and description. Matthew [1895b, PI. I] illustrated what appear to be two 

 dorsal valves, one somewhat more elongate than the other. With a large series of specimens 

 I am unable to find any ventral valve which shows the characters illustrated by Matthew 

 in his figure 5a [1895b, PL I]. On the contrary, most of them are to be observed in specimens 

 of the dorsal valve. 



Formation and locality. — Lower Ordovician: (114b )a Sandstone 1 mile {1.6 hm.) north of Lance Cove, Great 

 Belle Island, Conception Bay, Newfoundland. 



(326a [Grabau, 1900, p. 613]) Limestone pebbles in a Carboniferous conglomerate north of Fall River, Bristol 

 County, Massachusetts. 



MICKWITZELLA Walcott,6 subgenus of OBOLTTS. 



Aulonotreta Kutorga (in part), 1848, Verhandl. Russ.-kais. min. Gesell. St. Petersburg for 1847, No. 12, pp. 278-279. 



(Described and discussed in German as a new genus. The species referred to the genus belong with Oholus, 



Oholus (Acritis), and Oholus {Michwitzella).) 

 Oholus Eichwald, Davidson (in part), 1853, British Fossil Brachiopoda, vol. 1, introduction, No. 3, pp. 135-136. 



(Described and discussed. As far as they have been identified in this monograph the species referred to the 



genus belong with Oholus, Oholus (Acritis), and Oholus {Michwitzella).) 

 Not Thysanota Albers, 1860, p. 63. (Proposed for a genus of the Gastropoda; see Marshall's Nomenclator Zoologicus, 



p. 142.) 

 Oholus Eichwald (in part), 1860, Lethasa rossica, ancienne p&iode, vol. 1, sec. 2, pp. 924^925. (Described in French. 



The species referred to the genus belong with Oholus, Oholus {Acritis), and Oholus {Michwitzella).) 

 Oholus {Thysanotos) Mickwetz, 1896, M6m. Acad. imp. sci. St.-P6tersbourg, 8th ser., vol. 4, No. 2, pp. 194^195. 



(Characterized in German as a new subgenus. See below for translation.) 

 Oholus {Thysanotus) Mickwitz, Walcott, 1901, Proc. U. S. Nat. Mus., vol. 23, p. 683. (Characterized.) 

 Oholus {Michwitzella) Walcott, 1908, Smithsonian Misc. Coll., vol. 53, No. 3, p. 70. (Copies the original description 



of Mickwitz, as below.) 

 Oholus {Michwitzella) Walcott, 1908, idem, vol. 53, No. 4, PI. XI, and pp. 142 and 144. (Classification of genus.) 



The original description by Mickwitz follows : 

 The subgenus Thysanotos, containing a single species, 0. siluricus Eichwald, differs from the Cambrian subgenera 

 Euoholus and Schmidtia mainly by the fringed anterior border of the growth lamelte of its valves, and by the con- 

 centric striation arranged parallel to the posterior edge of these lamellae — two features that point to a peculiar organi- 

 zation of the edge of the mantle. The last-mentioned peculiarity appears also in the subgenus Acritis. 



Type. — Oholus siluricus Eichwald. 



The subgeneric name was given in honor of the late Dr. A. Mickwitz, of Reval, Esthonia. 



Obolus (Mickwitzella) siluricus (Eichwald). 



^•'' Plate XV, figm-es 1, la-c. 



Oholus siluricus Eichwald, 1843, Beitrage zur Kenntniss des russischen Reiches, Bd. 8, No. 2, pp. 7-8, PI. I, figs. 

 15a-c. (Described and discussed in German as a new species.) 



n 114b is the type locality, though the specimens in the United States National Museum to which that number is assigned were collected much 

 later than the type specimens. 



b The synonymy for this subgenus does not give a complete record of the various genera imder which the species now included in Mickwitzella 

 were formerly placed; it gives only those references in which the genus is discussed or described. To complete the record the following mera 

 generic references are listed: 



OboZMs Eichwald [1843, p. 7]. I OtoZits Schmidt [1861, p. 318; 1881, p. 17]. 



Obolus de Verneuil [1845, p. 290]. Zingula Schmidt [1881, p. 17]. 



OWits Eichwald [1859, PI. XXXVII, flgs. 6 and 7]. I OMiis Mickwitz [1892, p. 60]. 



