SIPHONOTKETID^. 615 



are very dissimilar in the two genera. It may be that with the discovery of good exteriors of 

 the ventral valve of Trematobolus excelsis Walcott that species will be found to have a circular 

 pedicle opening of the same character as that of Dearhornia clarTci, but from the similarity of 

 the cast of the interior of the ventral valve of the former species to that of Trematobolus Jcem- 

 panum (Matthew) it is referred to Trematobolus. 



The specific name is given in recognition of Dr. William B. Clark's work on the paleontology 

 of Maryland. 



Formation and locality. — Middle Cambrian: (9k) Limestone forming Ic of the Dearborn River section [Wal- 

 cott, 1908f, p. 201], on North Fork of Dearborn River, in the eastern part of the Lewis and Clark National Forest, 

 Lewis and Clark County, Montana. 



Genus TREMATOBOLtTS Matthew.^ 



[zpfiiia, perforation; and Obolus.] 



Trematobolus Matthew, 1893, Canadian Rec. Sci., vol. 5, pp. 276-279. (The species Trematobolus insignis is described 



and discussed, the description of the new genus and notes on its position being incorporated.) 

 Trematobolus Matthew, Hall and Clarke, 1892,6 Eleventh Ann. Rept. State Geologist New York for 1891, p. 252. 



(Described.) 

 Trematobolus Matthew, Hall and Clarke, 1892,6 Forty-fifth Ann. Rept. New York State Museum for 1891, p. 568. 



(Copy of preceding reference.) 

 Trematobolus Matthew, 1894, Trans. Roy. Soc. Canada for 1893, 1st ser., vol. 11, sec. 4, No. 8, pp. 88-90. (Except 



for the last paragraph, this is a copy of Matthew, 1893a, pp. 276-279.) 

 Trematobolus Matthew, 1895, Trans. New York Acad. Sci. for 1894-95, vol. 14, pp. 122-125. (A copy of the preceding 



reference.) 

 Protosiphon Matthew, 1897, Geol. Mag., new ser., dec. 4, vol. 4, pp. 68-69. (Described and discussed as a new genus.) 

 Protosiphon Matthew, 1898, Trans. Roy. Soc. Canada for 1898, 2d ser., vol. 4, sec. 4, No. 2, pp. 129-130. (Described 



and discussed.) 

 Trematobolus Matthew, Walcott, 1908, Smithsonian Misc. Coll., vol. 53, No. 4, PI. XI, and pp. 142 and 146. (Classi- 

 fication of genus.) 



Shell rather thick, subcircular or oval in outliae, biconvex; valves inarticulate. Ventral 

 valve with the beak at or very near the posterior margin. Pedicle opening in front of the 

 beak, at the anterior end of an elongate external groove that begins at the beak and broadens 

 out to the foramen, which is a simple opening through the thick shell; the elongate depression 

 and the opening through the shell are marked by strong hnes of sheU growth. Dorsal valve 

 usually more convex than the ventral ; beak marginal. 



Shell built up of several layers or lamellse that appear to be calcareocorneous, as the 

 result of the replacement in part of the original shell. Cardinal area of the ventral valve short; 

 it rises from the plane of the margins of the valves at angles rarying from 25° to 85° • it 

 varies in width and outline with the configuration of the valve. Area of dorsal valve nearly 

 on the plane of the margins of the valves or rising at angles as high as 15°. The area 

 of the ventral valve is divided midway by a distinct furrow that varies greatly in width 

 and depth; in some specimens it is little more than a faintly impressed line (PI. LXXXIII 

 fig. Ig) and in others it is an elongate triangular furrow (PI. LXXXIII, fig. li) ; in some speci- 

 mens the inner angle of the area next to the median furrow projects forward so as to form a 

 toothlike knob, which, in the cast, is shown by a deep, sharp indentation on each side of the 

 cast of the median furrow. These knobs are the teeth described by Matthew [1893a, p. 277]. 

 They correspond to similar projections in Obolella (PI. LV, figs, le. If, 3c), and Bicia gemma 

 (PI. L, figs. li-k). The area of the dorsal valve is narrow and divided by a central, slightly 

 depressed triangular space; the areas of both valves are crossed by strong, sharp, transverse 

 striae of growth that pass uninterruptedly across the median furrow. 



There are no traces of the foraminal groove on the interior of the ventral valve ; it is 

 entirely withm the substance of the shell and did not form a ridge on the inside of the shell. 

 A cast of the groove is shown in Plate LXXXIII, figure lo. 



The splanchnocoele of the ventral valve is back of the center, wliile in the dorsal valve 

 it extends to the center; a median depression extends nearly to the front of the splanclmoccele 

 of the ventral valve into which the foramen opens at about the posterior two-fifths of the 



a The synonymy for this genus includes only those references in which the genus is discussed or described. To complete the record Obolm 

 Matthew [1895a, p. 121; 1902c, p. 96] should be listed. 



!> This date (1892) is taken from the title-page and is used throughout this monograph, but the work is clearly antedated, the correct date being 



