The Fauna of the Maxi'illc Limestone. 385 



Uracliial valve most rotund on the umljo, with beak in- 

 ctirved 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 sintis 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 efifect 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 ch.aracterized b\' llie rounded foramen ; the 

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

 growth lines toward the anterior margin, tlius ])roducing a thick- 

 ened shell. 



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

 Alaxville 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. — Afaxville limestone. 

 Upper zone: Gladstone ^lills, Thompson Residence, White 



Cottage. 

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



Middle of Cut Xo. 6. Aloutli of Hough Hollow, Alt. Perry- 



Fultonham ; Kroft Residence. Kroft Bridge, White Cottage; 



Stimmel Residence, Maxville; Smith Chai:)el, Rogan. 

 Lower zone: liridge Gully, Cut Xo. 3, Cut Xo. 4, Mt. Perry- 



Fultonham. 

 Undetermined zone: Middle of Ctit X^o. 6, Mt. Perry-Fulton- 



ham; Harper .Shaft (?), Olive Furnace, 



