112 



Of these five horizons, one is in the Cold brook terrane, two in the 

 Lower and two in the 1 Upper Etchiminian faunal zones, and the gradual 

 increase in size is quite noticeable. As far as the assise E. 3 d, there is a 

 regular increase in the proportionate length of the valves, as compared 

 with their width. But the return to flaggy and silicious beds reduces 

 both the length and the superficial area of the valves. The layers of E. 

 3 e are crowded with multitudes of these little shells, which make up in 

 numbers what they lack in size ; while in the sandy layers of E. 2 (a ?) 

 the shells of this genus are scarce ; and notwithstanding the coarseness 

 of the matrix are thin. 



LEPTOBOLUS COLLICIA, PI. vi, figs. 3 a e. 



Leptobolus (?) collicia. Bull. Nat. Hist. Soc. N.B., vol. iv, p. 200, pi. i, figs. 3, ae. 



Leptobolus A small oval species somewhat pointed in front. Margins flattened, 

 especially in the posterior half. Remarkable for the peculiar spoutlike 

 flexure at the front of the dorsal valve, acquired in the later stages of 

 growth. A corresponding pair of grooves appears in the anterior part of 

 the ventral valve. 



Sculpture. This consists of fine concentric ridges, visible only with the 

 aid of a lens. These sometimes anastomose. 



Size. Length 10 mm., width 6 mm. 



The peculiarity from which the name of this species was taken has 

 been found to be a sporadic character ; many valves do not have it. 



The examples of this species first obtained, did not show clearly some 

 of the essential characters of Leptobolus and the generic reference was 

 left open. Examples obtained since then show that the conjectural 

 generic reference was a right one : such as the forward position of the 

 muscular scars and the grooves near the margin, supposed to be due to 

 vascular trunks : this added to the regular oval form, fine sculpture, low 

 hinge area, and similarity of the umbones of the two valves, are fair 

 characters of Leptobolus. 



Characters of Leptobolus collicia was a small species (though large for the genus), 

 ies ' moderately tumid, oval, but somewhat pointed in front ; margins flattened 

 somewhat in the posterior half. 



The ventral valve is pointed at the umbo, and has a very narrow apical 

 slit or groove ; it is evenly sloped to the margins, and, for a Leptobolus, 

 rather flatly arched. There are two furrows or a depressed area in some 

 examples, at the front of the valve. Interior. The callus is quite narrow, 

 subtriangular, and about three-sevenths of the length of the valve. The 

 lateral septa extend nearly half of the valve ; they reach beyond the callus. 



