131 



the height ; beaks at about one-sixth the length from the anterior extre- 

 rmty. Dorsal and ventral margins sub-parallel, slightly converging 

 posteriorly. Anterior extremity short, the margin broadly rounded up 

 to the hinge-line with which it appears to form a right angle. Outline of 

 the posterior extremity unknown, but apparently rounded. Ventral 

 margin gently convex. Both valves are moderately convex. A narrow 

 groove runs from the umbo backwards to the ventral margin, which it 

 reaches a little behind the mid-length. 



Surface on the anterior extremity with four or five strong concentric 

 ridges. These remain single to a point nearly beneath the beak when 

 their number is doubled by the intercalation of new ones, and apparently 

 by the fission of one or two of the old. Proceeding backwards the sur- 

 face markings become more and more obscure. The umbones are covered 

 with neatly rounded concentric ridges, about five in one line on approach- 

 ing the beaks, but becoming coarser, receding therefrom, down the sides. 

 A set of fine rounded stride radiate from the beak over the umbones, but 

 disappear at about half the height of the shell. On the umbones there 

 are five or six of those striae in the width of one line. 



The obHque sulcus running from the umbones to the ventral margin is 

 a character that would seem to ally this species to the genus Grammysia. 

 In Orthonota imdulata, (Conrad.) there is, however, a similar sulcus 

 in precisely the same position. 



Locality and Formation. — Arisaig, Nova Scotia. Upper Silurian. 

 Collector.— "I. C. Weston. 



Orthonota Simulans. (N. sp.) 



Plate 8, fig. 2. 



Description. — Transversely elongate, length about thrice the height^ 

 slightly convex ; dorsal and ventral margins parallel ; anterior extremity 

 short, the most projecting point at about half the height, thence broadly 

 rounded downwards and backwards to the ventral margin ; beaks at about 

 one fifth the whole length of the shell behind the anterior extremity ; 

 dorsal margin straight from the beaks backwards for about three fifths 

 the whole length ; posterior margin in the upper half, sloping at an angle 

 of about 130° with the dorsal ; lower half narrowly rounded. On the 

 umbones a flattening of the surface commences, which, gradually widening 

 and becoming slightly concave, extends to the ventral margin. Just 

 behind this, a moderately strong, broadly rounded angulation jextends 

 from the umbones backwards and downwards, to the lower posterior 

 angle. The dorsal margin is somewhat strongly compressed. 



