PLATE IV A. 



Legend, a. Deltidium. k. .Platform-vaults. 



b. Deltidial slope. m. Median .scars. 



c. Deltidial ridges. <!. Crown of the crescent. 



d. Ai-eal bordei'S. r. rSides of the crescent, 

 i. Umbonal chambers. t. Transverse scars. 



j. Platform. 



TEIMERELLA. Billings. 



Page 33. 



TlUMERELLA GKANUI.S, BillillgS. 



Fig. 1. An internal cast, viewed from the brachial side; showing the low umbonal chambej-s^ the length 

 of the platfoi-m-vaults and the lateral scars of the crescent. 



Fig'. 2. The reverse of the same specimen; showing the much greater prominence of the umbonal cham- 

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



the di'awings. . , i 



Niagara grouj). Near f^inking Spi'iny, Ohio. 



Trimerella Ohioensis, Mc^ek. 



Fig. 3. A small pedicle- valve ; showing a distortion of the umbo which extends to the posteiior portion 

 of the j)latforni. The drawing is made from a gutta-percha impressioii^ iand the shell is some- 

 what I'estoi'ed in the umbonal region. 



Niagara group. Port Bynm, Illinois. 

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

 crescent, the broad latei'al .scai-s of the cre.scent, the long, relatively narrow plattbi-m with its 

 medially depressed surface, and (he strong anterior septum. Drawn from a gutta-percha 

 impression of a natural cast. 



Niagara group. Rising Sun, Ohio. 

 Fig. 5. Cardinal view of an internal cast of both \-alves, the jiedicle-valve being above. The posterior 

 mai'gin of the brachial valve shows the impression of the umbo an<l beneath it a deep depres- 

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

 Fig. 6. Bi'achial view of the same specimen ; showing the diftei-ent lengths of the platfoi-m- vaults and the 

 deep umbonal chambei's of the opposite valve. 

 Niagara group. 6-awa, Ohio. 

 Fig. 7. View fi-om the bi'aehial side of a specimen retaining the shell ; showing also the cardinal area 



ami deltidium of the opposite valve. 

 Fig. S. The deltidium and cardinal area, drawn fi'om a gutta-percha impi'ession taken from the specimen 



represented in ligure'9. Slightly enlai-ged. 

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

 Niagara group. Ottawa county, Ohio. 



Trimerella Dalli, Davidson and King. 



Fig. 10. An internal cast of a pedicle-valve ; showing- the umbonal chaniliei-s, short platform-vaults and 

 indications of the transverse muscular scars. 

 Guelpli limestone. New Hope, Ontario. 



