OF CENTRAL CANADA PART IV. 



273 



FIG. 201. 



Cast of Trimerella 



acuminata 



(Billings.) 



Guelph Formation 



Family 1. Trimer e llidce : Shell, thick, calcareous, without fora- 

 men. Rudimentary hinge-teeth sometimes present. A broad? 

 bipartite plate, with supporting septum, in the interior of each valve- 



This family is entirely of Silurian age. It is represented chiefly 

 by the genera, Monomerella, with flat, central plate ; and Linobolus 

 and Trimerella, with the edges of the central plate 

 curved outwards. In all, the ventral valve has a 

 large area and deltidium, and the smaller valve is 

 nearly circular. Both valves are smooJi or with- 

 out external ribs, and moderately convex in form. 

 Several species of these genera occur in our Silurian 

 strata, more especially in the Upper Silurian 

 Guelph Formation. Examples are commonly in 

 the form of casts. These shew two or three deep 

 sinusses at or near the beak of the shell. 



Family 2. Craniadce : Shell thick, calcareous, finely-perforated, 

 more or less circular or square-shaped, without foramen. Beak of 

 both valves, sub central. 



This family dates from the Silurian period, but its typical genus 

 Crania is especially characteristic of Cretaceous strata. The internal 

 muscular impressions and presence of a nose-shaped septum produce 

 the appearance of a face or skull whence the name of the genus. 

 Our strata are without recognized representatives of the family. 



Family 3. Discinidce : Shell composed of horny and calcareous 

 layers, more or less circular in form, with central or sub-central beak, 

 and foramen (usually in the form of a narrow slit) in the larger valve. 



This family is represented chiefly by Discina, Orbiculoidea, and 

 Tr emails. The horny part of the shell in these genera (as in the 

 lingulidce) is composed essentially of calcium 

 phosphate. Examples, mostly of small size 

 occur in our Silurian strata. The genus, 

 Discina has living representatives. The other 

 genera are extinct. 



Family 4. Lingulidce : Shell composed of 

 horny and calcareous layers, the horny por. 

 tion consisting of calcium phosphate. Com- 

 monly oblong or oval in shape, more rarely 

 18 



FIG. 202. 

 Discina (or Orbiculoidea) 



(.'//(< t 

 Trenton Formation. 



