PLATE IV A. 



Legend, a. Deltidium. k- Platfoim-vaulte. 



b. Deltidial slope. "i- Median scars. 



c. Deltidial ridges. '1. Ci-owu of the crescent. 



d. Areal bordei'S. r. Sides of the crescent. 

 Unibonal chambei-s. t. Transverse scars. 



1. 



j. Platform. 



THIMEKELLA, Billings. 



1)-JI il: - ' .■ .  : 



Page 33. 



Trimerklla grandis. Billings. 



Pig. 1. An internal cast, viewed from the brachial side; showing the low unibon.il 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 groatei- prominence of the \nnbonal cham- 



bei-3 in this valve. The anterior margin and casts of the vaults have been slightly re.stored in 



ttie drawings. 



Niagara group. Near Sinking Spring, Ohio. 



Trijikrella Ohioensis, Meek. 



Fig. 3. A small pedicle-valve ; showing a distoj'tiou of the umbo wluch extends to the posterior portion 

 of the platform. The di'awing is made from a gutta-percha impression, and the snell is some- 

 what restored in the umbonal I'cgion. 



Niagara group. Port Byron, Illinois. 



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

 crescent, the broad lateral scars of the ci-escent, the long, relatively nari-ow plattbrm with its 

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

 impression of a natural cast. 



Niagara group. Rising San, Ohio. 



Fig. .1. Cardinal view of an intei-nal cast of lioth valves, the pedicle-valve being above. The posterior 

 margin of the brachial valve shows the impression of the umbo and beneath it a <leep depres- 

 sion representing the cresi^ent ; the deltidial slope of the pedicle-valve is also visible. 



Fig. ti. Brachial view of the same specimen ; showing the diffeient lengths of the platform-vaults and the 

 deep umbonal chambers of the opposite valve. 

 Niagara group, ffenoa, Ohio. 



Fig. 7. View from the brachi.il side of a specimen retaining the shell ; showing also the cardinal area 

 and deltidium of the opposite valve. , 



Fig. 8. The deltidiiim and cardinal area, drawn from a gutta-percha impressioti tiiken ft'om the specimen 

 repre.sented in figure 9. Slightly enlarged. 



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

 Niagai'a group. Ottawa county, Ohio. 



Tkimerella Dalli, Davidson and King. 



of a pedicle-valve ; showing I 

 the transverse muscular scars 

 Guelph limestone. iVisiu Hope, Ontark). 



Fig. 1(1. All iTiti-rnal cast of a pedicle-valve ; showing the umbonal chambers, short platform-vaults and 

 indications of the transverse muscular scars. 



