TJic Fauna of the Maxvillc Limestone. 385 



Brachial valve most rotund on the umbo, with beak in- 

 curved beneath the one on the other valve. Mesial fold promi- 

 nent, rapidly increases in size toward the anterior end. due to 

 the sudden development, on either side, of a sinus which cor- 

 responds to the two lateral folds of the opposite valve. A lateral 

 fold on both sides of the valve and beyond each lateral sinus. 

 The median fold with this lateral fold on either side gives a 

 strong trilobate effect to this valve. 



Surface marked by concentric lines of growth which are,, 

 in most specimens, crowded toward the anterior end. 



Length i6 mm. ; width i8 mm. 



The shell is characterized by the rounded foramen ; the 

 trilobate, brachial valve ; and the crowding of the concentric, 

 growth lines toward the anterior margin, thus producing a thick- 

 ened shell. 



This species is one of the four most abundant forms in the 

 Maxville limestone. It occurs in great numbers in the shale- 

 nodular zone and rather abundantly in the shaly partings of the 

 upper half of the stratum. It separates quite readily from the 

 shales, and its greater abundance in these is due to the greater 

 destruction of life during these intervals caused, perhaps, by the 

 clay impurities. 



Horizon and locality. — Maxville limestone. 

 Upper zone : Gladstone Mills, Thompson Residence, White 



Cottage. 

 Shale-nodular zone : Cut No. 5, Upper end of Cut No. 6, 



Middle of Cut No. 6, Mouth of Hough Hollow, Mt. Perry- 



Fultonham ; Kroft Residence, Kroft Bridge, White Cottage ; 



Stimmel Residence, Maxville ; Smith Chapel, Logan. 

 Lower zone: Bridge Gully, Cut No. 3, Cut No. 4, Mt. Perry- 



Fultonham. 

 Undetermined zone : Middle of Cut No. 6, j\It. Perry-Fulton- 

 ham ; Harper Shaft (?), Olive Furnace. 



