82 



The beak of the ventral valve exhibits in the cast a very short fissure, 

 seldom more than two lines in depth, indicating that the mesial septum 

 was short and thick with, sometimes, a rounded edge. One specimen 

 shows that the tri-angular chamber, at its lower angle, projected far 

 into the cavity and apparently almost reached the shell of the dorsal 

 valve. 



The casts of the interior of the dorsal valve show at the beak a short 

 roof-like ridge, about two lines in length, and about the same in width. 

 This seems to prove that in the hinge-line of the perfect shell there is 

 ' an anovular notch or tri-an^^ular foramen. On each side of the rido:e 

 there is seen one of the small pits occupied by the crural processes. 



No traces of the muscular impressions of the ventral valve are visible 

 in any of our specimens. In the dorsal valve they are small (in propor- 

 tion to the large size of the shell), and situated immediately below the 

 beak. They are of an ovate or oblong shape, divided along the middle. 

 In a specimen four inches in length the scars are seven lines in length 

 and five lines in width. 



The concentric markings, on the surface, show that the shell was 

 subject to occasional interruptions of its growth. They also give very 

 distinct outhnes of the form at different ages, from that of the young shell, 

 one inch in length, up to the old ones, of five inches. 



The radiating ribs along the middle proceed straight to the front, but 

 at the sides they curve outwards, those near the hinge line reaching the 

 margin at about a right angle to the length of the shell, sometimes even 

 slightly curved upwards. They are, as seen in the cast, low, rounded, 

 usually about one line or a little less. They appear to increase by bifur- 

 cation and intercalation. In most specimens, they are crossed by obscure 

 concentric crenulations. 



Length of large specimen, four or five inches. The width appears to be 

 a' little greater. Depth of both valves, one inch and a half. 



This species is closely allied to the English S. Uratus, but is a much 

 larger shell. In a collection of fossils from Gothland, sent to the Survey 

 by Dr. Lindstrom, there are three specimens of the Swedish form that 

 have been referred to S. liratiis. They are decidedly distinct from our 

 shell. 



Locality and Formation. — Near Thorold, Ontario, in the Chnton for- 

 laation. 



Collectors. — A. Murray, E. Billings. 



