OBOLID^. 



571 



circular, scarlike spots, one on each side of the median line and just in front of the area, that 

 recall in appearance and position the posterior adductor scars of Crania. The ventral valve is 

 so much like that of B. gemma (Billings) that it is difficult to decide whether such shells as are 

 represented by Plate L, figures 2a and 2b, should not be referred to B. gemma. There is a 

 gradual transition in form and character of the interior of the ventral valve between the 

 extremes represented iu B. gemma b}' figures Ih and li, through figures If, Ig, Ik, 2a, and 

 2b, to the extreme form of B. wJiiteavesi, figure 2c. If it had not been for the bosses in figure 

 2c, and the dorsal valve, figure 2e, associated with it, I would have hesitated to refer it to a 

 distinct species. 



The specific name is given in honor of Prof. J. F. Whiteaves, of the Geological Survey 

 of Canada. 



Formation and locality. — Lower Cambrian: (2b) Limestone just north of Beman Park, in the northeastern 

 part of the city of Troy, Troy quadrangle (U. S. Geol. Survey), Rensselaer County, New York. 



Genus DICELLOMUS Hall.a 



Dicellomus Hall, 1873, Twenty-third Ann. Kept. New York State Cab. Nat. Hist., p. 246. (Characterized as a new- 

 genus.) 



Schmidtia Zittel (in part) [not Volboeth], 1880, Handbuch der Palaeontologie, Bd. 1, Abth. 1, p. 665. (Doubtfully 

 places Dicellomus as a synonym of Schmidtia.) 



Obolella Billings, Hall and Clarke (in part), 1892, Eleventh Ann. Kept. State Geologist New York for 1891, pp. 

 240-241. {Dicellomus is included with Obolella as a synonym.) 



Obolella Billings, Hall and Clarke (in part), 1892, Forty-fifth Ann. Kept. New York State Museum for 1891, pp. 

 556-557. (Copy of preceding reference.) 



Obolella Billings, Hall and Clarke (in part), 1892, Nat. Hist. New York, Paleontology, vol. 8, pt. 1, pp. 66-73, espe- 

 cially pp. 72-73. {Dicellomus is discussed as a synonym of Obolella.) 



Dicellomus Hall, Walcott, 1899, Mon. U. S. Geol. Survey, vol. 32, pt. 2, p. 446. (Discussed, part of the paragraph 

 being copied below, see p. 572.) 



Dicellomus Hall, Walcott, 1905, Proc. U. S. Nat. Mua., vol. 28, pp. 312-313. (Described and discussed essentially 

 as below.) 



Dicellomus Hall, Grabau and Shimer, 1907, North American Index Fossils, vol. 1, p. 189. (Described.) 



Dicellomus Hall, Walcott, 1908, Smithsonian Misc. Coll., vol. 53, No. 4, PI. XI, and pp. 142 and 144. (Classification 

 of genus.) 



Shell small; general form ovate to subsemicircular, biconvex, with apices marginal. Sur- 

 face of outer shell finely punctate in all species where it is preserved uninjured. Interior or 

 middle lamellae marked by radiating striae and minute punctse; inner sui'face finely punctate. 

 The shell is thick in all the species now known, and is built up of a thin, outer, scabrous layer, 

 numerous inner layers or lamellae, and a thin inner layer. Shell substance apparently calcareo- 

 corneous. 



The interior of the ventral valve (PI. LII, figs. Ic, Id) shows a short area with a median 

 pedicle groove; an elongate visceral ai'ca; well-marked main vascular sinuses; large, composite 

 scars (cl) where the posterior muscles, i. e., transmedian and outside and anterior laterals, were 

 attached; and a short shelf that extended into the valve from each side of the pedicle groove. 

 This extension appears to correspond to the dental plate in the articulate brachiopods, and 

 it has a shght thickening at the anterolateral margin that suggests a short tooth. In material 

 received from Prof. W. A. Finkelnburg from Hudson, Wisconsin, I worked out several interiors of 

 the ventral valve and found in all of them more or less of the remains of a lining of the pedicle 



a The synonymy for this genus does not give a complete record of the various genera under which the species now included in Bkellomu? were 

 formerly placed; it gives only those references in which the genus is discussed or deserihed. To complete the record the following mere generic 

 references are listed: 



Obolus Owen [1852, p. 501]. 



Lingula Hall [1861, p. 24]. 



Obolella Billings [1861b, p. 7; 1861c, p. 946]. 



Lingula Hall [1862, p. 435]. 



06o7c!ZaBillings[1862c,p.421;1862d,p.67;1862e,p.218]. 



Obolella Meek and Hayden [1862, p. 435]. 



Obolella Hayden [1862, p. 73]. 



impata ■Whitfield [1862, p. 136]. 



Obolella? HaU [1863, p. 133]. 



Lingulepis Meek and Hayden [1865, p. 3]. 



Obolella Meek and Hayden [1865, p. 4]. 



Obolella ? HaU [ 867, p. 112]. 



Obolus I^Tiitfield [1875, p. 103]. 



Obolus ? AVhitfield [1880, p. 338]. 



Obolella 'Whitfleld [ISSO, pp. 339 and 340]. 



Obolella Walcott [1886b. p. 111]. 



Obolella Schuchert [1897, p. 275]. 



Dicellomus Walcott [1901, p. 673; 1908d, p. 77]. 



