106 



etc., though seeming to prefer a sandy bottom, in clear quiet water 

 They abound in the Upper Etcheminian, and are spread vertically from 

 the Coldbrook terrane to the summit of the Cambrian. 



LEPTOBOLUS ATAVUS, PI. VI, figs. 2 a g. 

 Leptobolus L. atavuft, n. sp. Bull. Nat. Hist. Soc., New Brunswick, vol. iv, p. 



atavus. onA i .c r> s 



200, pi. 11, figs. 2, a to/. 



A synopsis of the description of this species was published in the Bul- 

 letin of the Natural History Society of New Brunswick as above noted, 

 a fuller description is the following : 



Shell substance thin, calcareo-corneous. Valves somewhat quadrately 

 oval to ovate ; rather strongly arched transversely and behind. 



Ventral valve somewhat pointed behind and having an inconspicuous 

 umbo. Cardinal area very small and obscure, not more than one-seventh 

 of the length of the valve. No pedicle groove has been determined, but the 

 shell deepens suddenly in the interior behind the cardinal line. Except at 

 the umbo the margins of the shell are strongly arched down. The interior 

 shows at the side a pair of long lateral scars in the posterior half of the valve; 

 extending from these scars forward are arching impressions of the vascular 

 trunks. The scars of the central muscles are in the front half of the valve. 



The dorsal valve differs from the ventral in the rounded posterior end 

 and in the depressed longitudinal band which traverses the median part. 

 The interior has a rhombic depression at the back, on the sides of which 

 are placed the prints of a pair of muscle scars. Thence a narrow raised 

 band extends along the median line nearly to the front. On each side near 

 the middle of this band is a small muscle scar. Impressions of a vascu- 

 lar band extend along each side of the valve. 



Sculpture. Somewhat obscure, fine concentric striae, visible with a 

 lens, can be observed on all parts of the valve ; there are about 15 to 20 

 in the spaoe of a millimetre. But a strong lens will hardly resolve the 

 finer ornamentation, which consists of a granulation, due to opposite 

 series of striae, crossing the shell diagonally. The second layer of the 

 shell is smooth and shining, but having concentric ridges similar to those 

 of the outer layer, without the minuter markings. Stronger growth 

 lines occur at intervals of 6 to 10 of the concentric ridges. 



Size. Length, 5 mm ; width, 3 mm. ; depth, 1 mm. Young indi- 

 viduals are proportionately wider, because in the late? stages of growth 

 additions are made chiefly to the front of the valve. 



Horizon and locality. In assises E. 3 d and e (Upper Etcheminian) 

 at Dugald brook, Escasonie, &c. 



