110 



Mutation 

 insulse. 



Compared to 



Lingulella 



linguloides. 



LEPTOBOLUS atavus, mut. INSULSE, n. sp. Plate VI, figs. 4, a c. 



Minute, shell thin, corneous or calcareo-corneous. Valves deep, elongate 

 oval, having the edges rolled back. Ventral valve more pointed than the 

 dorsal, having a small cardinal area where the shell is thickened, about 

 one-tenth of the length of the valve. Interior. There is a lenticular 

 cavity just in front of the hinge line. Visceral callus extending nearly 

 to the middle of the valve ; a faint transverse swelling is seen on some 

 valves, half way from the beak. Grooves, that appear to be due 

 to vascular trunks, but run along near the lateral margins of the valve. 

 The dorsal valve approaches an elliptical form. It has a short area, 

 with a tubercle on the axial line. Interior. In front of the area is a 

 transverse depression in which on each side is a pit, apparently for the 

 attachment of the posterior adductors. The depression merges on each 

 side into long, narrow lateral grooves (for vascular trunks ?) like those of 

 the ventral valve. The visceral callus extends across the middle third of 

 the valve, where also on the axial line are low septal ridges. 



Sculpture. This species has fine striae, concentric to the umbo, over 

 the whole shell. 



Size. Length of ventral valve, 

 mm. shorter. 



mm. ; width 2^ mm. ; the dorsal is 



Horizon and locality. Assise E. 2, (a 1) Lower Etcheminian at Young's 

 Point. George R. Station, N. S. Scarce. 



This form is of about the same length as Lingulella linguloides of the St. 

 John terrane in New Brunswick (which also appears to be a Leptobolus), 

 but is proportionately narrower ; it also resembles L. atavus of the 

 Upper Etcheminian Fauna in form, but is considerably smaller and nar- 

 rower and the callus of the ventral valve is proportionately shorter. 

 The shell is very thin, and is flexible, as its valves are more distorted 

 than those of the Lingulellas with which it is associated, and it is often 

 moulded on the grains of sand with which it is imbeded. When the 

 shell is distorted diagonally it may be mistaken for a small Ostracod, 

 owing to the vascular groove which gives the margin the appear- 

 ance of having a marginal fold, so often found in this group of crus- 

 taceans. 



The following are dimensions of the valves of this mutation. 



