PLATE IV a. 
Legend. 
a. Deltidium. 
b. Deltidial slope. 
c. Deltidial ridges. 
d. Areal borders. 
i. Umbonal chambers 
j. Platform. 
k. Platform-vaults, 
m. Median scars. 
q. Crown of the crescent. 
r. Sides of the crescent, 
t. Transverse scars. 
TKIMERELLA, Billings. 
Page 33! 
Trimerella grandis. Billings. 
Fig. 
Fig. 
Fig. 
Fig. 
Fig. 
Fig. 
■Fig. 
Fig. 
1. An internal cast,' viewed from the brachial side ; showing ! th&- low umbonal chambers, the length 
of the platform-vaults and the lateral scars of the crescent. 
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' vaultsihave been slightly restored in 
. the drawings.. 
Niagara group. Near Sinking Spring, Ohio, . , 
Trimerella Oiiioensis, Mock. 
3. A small’ pedicle-valve ; showing a distortion of the umbo which extends to the posterior portion 
of the platform. The drawing is made front 1 a gutta-percha-impression, and- the : shelf is some¬ 
what restored in the umbonal region. 
Niagara group. Port Byron, 1 Uinois. 
4. A brachial valve ; showing the incurvature of the beak, the'&uburnbonal ridge merging into the 
crescent, the broad lateral scars of the crescent, the long, relatively narrow platform with its 
medially depressed ■ surface, ‘ and the strong anterior Septum. Drawn'from a gutta-percha 
impression Of a natural cast. 
Niagara group. Rising Sun, Ohio. 
'). Cardinal view of an internal cast of both valves, the pedicle-valve being above. The posterior 
margin of the brachial valve shows the impression of tlie umbo and beneath it a deep depres¬ 
sion representing the crescent; the deltidial slope of the pedicle-valve,is also visible. 
6. Brachial view of the same specimen ; showing the different lengths of the platform-vaults and the 
deep umbonal chambers of the opposite valve. 
Niagara group. Genoa, Ohio. 
7. View from the brachial side of a specimen retaining tlie shell; showingdilso the cardinal area 
and deltidium of the opposite valve. 
8. The deltidium arid cardinal area, drawn frbiri a gutta-perclia impression taken from the specimen 
represented in figure 9. Slightly enlarged. 
9. An internal cast of the pedicle-valve of a large individual. 
Niagara group. Ottawa county, Ohio. 
Trimerella Balli, Davidson-and King. 
Fig. 10. An internal cast of a pedicle-valve; showing the umbonal chambers, short platform-vaults and 
indications of the transverse muscular scars. 
Guelph limestone. New Hope, Ontario. 
