72 MONOMYARIA. ORTHIS. 
Genus VII. — MAGUS. — Sowerby. 
Generic Character. — Shell inequivalve, equilateral ; one 
valve convex, provided with an angular sinus along an in- 
curved beak ; line of the hinge, and back of the other valve 
straight, with two projections near the centre; a partial 
longitudinal septum attached to the hinge within. 
Magus plumulus. Plate VII. fig. 15. 
This genus has much the general form of some of the smooth Terebratule, 
but will readily be distinguished from the shells of that genus by an examina- 
tion of the parts about the hinge. The beak is destitute of perforation, but 
there is a large quadrangular foramen, two sides of which are formed by two 
ridges from the straight back of the depressed valve, and other two run along 
to the point where they meet; on the side of it there is a flattened space, 
emanating from the hinge line, which is considerably longer than the foramen, 
and terminates in the apex. On separating the valves the foramen is divided 
into two angular sinuses, that in the beaked valves being considerably larger 
than the other. A thin longitudinal septum is situate in the middle of the 
shell, reaching from one valve to the other, the upper part of it extending in 
an arcuated form over the hinge, and having a perpendicular front, with two 
shelf-like appendages, placed one above another, the upper ones attached by 
slender testaceous processes to the hinge. 
Known only in a fossil state, and is found in the Chalk near Maudesley, 
Norwich, and but one species has been discovered. 
Genus VIII. — TRIGONOSEMUS. — Konig. 
Generic Character. Shell inequivalve, equilateral; sub- 
trigonal; one valve generally more convex than the other; 
one of them prolonged into a lengthened beak, truncated 
at the point, and perforated for the passage of a tendon, by 
which the animal attached itself to extraneous bodies; 
hinge destitute of a ligament, but provided with two teeth, 
in one valve, which lock into corresponding cavities in the 
other; two muscular impressions, situate near the centre of 
both valves. 
Trigonosemus lyra. Plate XI. fig. 15. 
Found in the Greensand of Chute Farm, near Horningsham. 
This is exclusively a fossil genus; which is formed of Terebratula lyra of 
Sowerby, and other corresponding species. 
Genus IX.— ORTHIS. — Dalman. 
Generic Character.— Shell inequivalve, with a recti- 
linear hinge; umbones distant ; larger valve with a trans- 
verse smooth area, and a triangular pit. 
Orthis canalis. Plate XI. fig. 14. 
Found in the Wenlock Shale, which is a member of the Silurian series, 
