82 AMERICAN JOURNAL 



at its lateral edges, and more deeply grooved over the septum, 

 so that the apparatus presents the appearance of a pair of cylin- 

 ders side by side, whose anterior edges are more or less flexuous, 

 and with the septum longer and more prominent than in the 

 neural valve. Shell calcareous, with concentric lines of growth, 

 apex rather pointed, anterior margin broadly rounded, without 

 a foramen, hinge teeth or interlocking processes. 



The Gotland specimens are too poor to state whether these 

 characters are common to them. It is my impression that they 

 will prove to be so, notwithstanding Dr. Lindstrom's figures. 



Type T. grandis, Billings, op. cit. p. 166, f. 151, a, 6, 1862. 



Characterized by a nearly straight hinge-line slightly flexuous 

 in front, and by the large cavities below the neural lamina, not 

 reaching the beaks. 



Locality. Casts found in the Guelph limestone, middle Silu- 

 rian, at Gait, New Hope and Elora, J. Richardson, J. Dalgleish, 

 R. Bell and E. Billings. 



T. acuminata, Billings, op. cit. p. 167, fig. 152. PI. , fig. 4-5. 

 Ibid. Am. Journ. Sci. Arts, 3d series, i, p. 471, June, 1871. 



Characterized by the extension of the septum and cavities of 

 the lamina into the beaks below and behind the hinge margin, 

 which is narrow and straight. The valve below the lamina ap- 

 pears to have been somewhat thickened, so that the under sur- 

 face of the cavities was raised above the body of the valve. 



The shell represented by figures 4 and 5 is referred by Mr. 

 Billings to this species. 



Locality, with the preceding. 



T. Billingsi, n. s. Plate 11, fig. 1, 2, 3. 



Characterized by the lamina in the neural valve being hardly 

 raised above the shell except at the anterior extremity ; the sep- 

 tum hardly extending beyond the end of the lamina anteriorly ; 

 the area with a broad depressed transversely striated space in 

 the middle, on each side of which are two longitudinal ribs or 

 elevations, separated by a shallow gutter, across all of which the 

 strige of accretion pass without interruption ; margins flat and 

 nearly straight. Posterior margin slightly concave in the middle. 



Locality. Guelph limestone, E. Billings. 



One cast of a neural valve, showing the characters of the area, 

 posterior margin, &c, distinctly, was kindly lent for examination 

 and description by Mr. Billings. I have also seen two other 

 specimens from the same locality. 



