PLATE IV A. 



Legend. a. Deltidium. k. Platfoi-m-vaults. 



b. Deltidial slope. m. Median scars. 



c. Deltidial ridges. q. Crown of the crescent. 



d. Areal borders. r. Sides of the crescent, 

 i. Unibonal chambers. t. Transverse scars. 



j. Platform. 



TRIMERELLA, Billlng.s. 



P.age 33. 



Teimerella grandis. Billings. 



Fio-. 1. An internal cast, viewed fi-om the brachial side ; showing- the low unilional chambers, the length 

 of the platform-vaults and the lateral scars of the crescent. 



Fi". 2. The reverse of the same specimen; showing the much greater jirominence of the umbonid cham- 

 bers in this valve. The anterior margin and casts of the vaults have been slightly restoi-ed in 

 the di-awings. 



Niagara group. Near /Sinking Spring, Ohio. 



Trimerella Ohioensis, Moek. 



Fig. 3. A small pedicle-valve ; showing a di.stortion of the umbo which extends to the posterioi' portion 

 of the platform. The drawing is made froni n gutta-percha impression, and the shell is some- 

 what restored in the umb(nial i-egion. 



Niagara group. Port Byron, I llinhisi^ ' ,!,';',' -rM-,',V.! 



Fig. 4. A brachial valve ; showing the incurvature of the beak, the subumbonal ridge merging into the 

 crescent, the broad lateral scars of the crescent, the long, relal.i\ely narrow platform with its 

 medially depressed surface, and the sii-ong .anterioi- septum. Drawn from a gutta-pei'cha 

 impression of a natural ca.st. 



Niagara group. Rising Sun, Ohio. 



Fig. ."). Cardinal view of an internal cast of both valves, the pedicle-valve being above. The posterior 

 margin of the bi'achial valve shows the impres.sion of the umbo and beneath it a deep depi-e.s- 

 sion representing the crescent ; the deltidial slope of the pedicle-valve is also visible. 



Fig. •). Brachial view of the same specimen ; showing tlie dilferent length.s of the (ilatfoi'm- vaults and the 

 deep umbonal chambers of the opposite valve. 

 Niagara group. Qenoa. Ohio. 



Fig. 7. View from the br.achial side of a specimen retaining the shell ; showing :dso the cardinal aiva 

 and deltidium of the ojiposite valve. 



Fig. S. The deltidium and cardinal area, drawn from a gutta-percha impression taken from the sjiecimen 

 represented in tigure 9. Slightly enlarged. 



Fig. 9. An internal cast of the pedicle-valve of a large indiviiUud. 

 Niagara group. Ottawa county, Ohio. 



Trimerella Dalli, Davidson and King. 



Fig. 10. An internal cast of a pedicle-v.alve ; showing the umbonal chamber.?, short jilatform-vaulfs and 

 inilications of the transverse muscular scars. 

 Gnelph limestone. New Hope, Ontario. 



