190 



Sculpture. No examples have been obtained showing the surface mark- 

 ings, except those near the front of the valve ; but both valves show from 

 four to six ridges of growth in the anterior quarter of the valve ; these ridges 

 are broadly curved in the middle part, but more abruptly at the sides. 



Size. Ventral valve 4 mm. long and 2 mm. wide. The dorsal valve is 

 nearly one half millimetre shorter than the ventral. 



Compared Horizon and locality. In grey flags of the Middle Cambrian (Div. 



with L.msulse. -.-. /.- XT M i i / i 



2 c.) on the eastern slope or tae valley or McWeil brook (on road to 



Trout brook) Mira river, N.S. 



This species is of nearly the same size as L. insulce, but is less elliptical 

 in outline, and the valves are more flattened in front ; the area of the ven- 

 tral valve also is longer, and that of the dorsal shorter than in that species ; 

 also the central group of muscles is not set so far forward as in that of 

 the dorsal of the species cited. 



This species differs from L. atavus of the Etcheminian terrane in having 

 the central muscles of both valves set farther back. 



LEPTOBOLUS GEMMULUS, PI. XIV., figs, la to c. 



Obtlella ? gemmula, Trans. Roy. Soc., Can., vol. ix, sec. iv, p. 41, pi. 

 xii, figs. 8a-c. 



L tobolus The following is the original description of this species : 



described. Ovate acuminate, sides somewhat straight in the posterior half, 



rather broadly rounded in the anterior half of the shell. Test, calcareo 

 corneous. 



Ventral valve acuminate behind ; it has a small triangular area, with 

 pedicle groove and low beak. The interior of the valve has a ridge, most 

 distinct in the posterior third, along the median line ; this part of the 

 ridge appears double in some shells, but in others there is only a single 

 sharp ridge. On each side of this part of the ridges, but not extending 

 its full length, are suboval scars of the adductor muscles. The central 

 part of the shell is occupied by a large bilobed sub-circular scar (?, rather 

 depression). The outer borders of these two depressions of the shell are 

 defined by a sharp line, elevated in the middle of its length, and there 

 bent toward the centre of the shell. The margin of the valve appears 

 flattened and crenulated. 



The dorsal valve is blunt and depressed at the umbo. Interior smooth 

 except at the posterior end, where concealed in the blunt beak, is a short 

 ridge, dividing the small scars of the (posterior) adductor muscles; a 

 small scar on each side of the valve close to the edge, and about one 



