SIPHONOTRETID^. 621 



The specific name was given in honor of Prof. J. F. Kemp, of Cohimbia University, New 

 York City. 



Formation and locality. — Middle Cambrian: (2u) Lowest beds exposed on the south side of Long Island; and 

 (308g) shales of Division lb3 or lh4 [Matthew, 1897, p. 71] of Matthew's section on Long Island; both in Kennebecasis 

 Bay [see Matthew, 1898a, pp. 124 and 127], St. John County, New Brunswick, Canada. 



Trematobolus pristinus (Matthew). 



Plate LXXXIII, figures 2, 2a-h; Plate LXXXIV, figure 6. 



Obohis pristinus Matthew, 1895, Trans. New York Acad. Sci. for 1894-95, vol. 14, pp. 121-122, PI. IV, figs. la-d. 

 (Described as a new species. The specimen represented by figui-e Ic is redrawn in this monograi^h, PL 

 LXXXIII, fig. 2f.) 



Obolus pristinus Matthew, 1902, Trans. Roy. Soc. Canada, 2d ser., vol. 8, sec. 4, No. 3, p. 96, PL I, figs. 8a-d. (Char- 

 acterized. Figs. 8a-d are copied from figs, la-d, respectively, of the preceding reference.) 



General form biconvex, subcircular to short ovate. Ventral valve rounded acuminate, 

 sometimes a httle broader than long; beak marginal above a low area. Pedicle opening at 

 the anterior end of a furrow thikt gradually narrows to the beak; the foraminal furrow varies 

 in length from about one-third to two-fifths the length of the valve; it is situated in the thick- 

 ened part of the shell and does not produce a ridge on the interior of the valve. Dorsal valve 

 proportionally a httle shorter than the ventral; beak marginal; area, if present, unknown. 



Surface marked by fine concentric striae and fines of growth that show as the front margins 

 of imbricated lamellae of the shell; very fine radiating striae (also a few radiating, slightly 

 elevated fines) occur on some specimens, but whether they belong on the epidermal layer or 

 one of the inner lamellae is unknown; the inner surface is sfightly roughened. The substance 

 of the shell has been replaced by the calcareous matter of the matrix, but from the calcareo- 

 corneous nature of the shell of T. Icempanum (Matthew) it is probable that the original sheU 

 of this species was of the same nature. The shells vary in size from 5 to 13 mm. in length; the 

 average is about 9 mm. 



The area of the ventral valve is short; otherwise, nothing is known of it. The splanch- 

 nocoele extends nearly to the center of the valve; a low ridge starts near the area, and, gradu- 

 ally widening, passes beyond the pedicle opening into an oval depression forming the anterior 

 portion of the visceral cavity; the ridge back of the pedicle opening is marked by a slightly 

 raised median hne; the space on each side, between the median ridge and the main vascular 

 sinuses, has a narrow band across the front that extends from each side of the central depressed 

 area diagonally backward to the main sinuses; these bands correspond in position to the spaces 

 occupied by the central muscle scars and the transmedian and outside lateral scars (compare 

 PI. LXXXIII, fig. 2b, with PL VII, fig. 13, of Oholus); the anterior lateral muscle scar is close 

 to the main vascular sinus, nearly opposite the pedicle opening. The main vascular sinuses 

 start near the area on each side of the low median ridge, and, with a gentle outward curve, 

 pass the center, and then, with a slight inward curve, reach nearly to the front margin; they 

 do not extend laterally beyond a vertical line drawn midway between the center and lateral 

 margins of the valve. Fine, irregular, secondary radial canals extend outward from the main 

 canal toward the margins. 



The splanchnocoele of the dorsal valve extends beyond the center; a very narrow median 

 septum is shown, or else a broad, low, rounded ridge. The main vascular sinuses start near 

 the median line, close to the posterior margin, and curve forward, subparallel to the lateral 

 margins of the valve, gradually getting farther away from the margins as they pass the trans- 

 verse center of the valve. The central muscle scars are elongate oval in outline and situated 

 back of the center of the valve; the anterior laterals are small and close to the median fine. 



Notliing is clearly shown of the details of the area of the dorsal valve; it was probably 

 similar to that of T, kempanum (Matthew). (Compare figs. 2f and 2g with figs. 1 1 and Im, PI. 

 LXXXIII.) 



Observations. — This species is closely related to T. insignis Matthew, reference to wliich 

 is made under that species; it differs in the position of the main vascular sinuses of the ventral 



