15 



Ilionia galtensis. (N. Sp.) 

 PlateS, figs. 1.1a & 16. 



Shell compressed, sinuated, most convex in the direction of a line 

 which might be drawn from the beaks to the centi-e of the ventral mar- 

 gin, behind which faint prominence there is a broad, shallow depression, 

 bounded posteriorly by an oblique and somewhat curved keel or naiTOW 

 ridge, which extends from the beaks to the posterior end of the ventral 

 margin, and marks out a laterally compressed posterior area. Length 

 about one third (or less than one third) greater than the height ; 

 anterior end broader than the posterior, rounded and somewhat ex- 

 panded at its upper and lower margins : posterior end narrowing above 

 and below, and truncated almost vertically but somewhat concavely at 

 its extremity. Dorsal margin nearly straight or slightly convex, and 

 sloping very gently downwards behind the bealis, concave and some- 

 what ascending in front of them : ventral margin convex anteriorly, 

 narrowing rather rapidly and concavely upwards posteriorly. 



TJmbones rather broad, subcentral and carinated behind : beaks small, 

 appressed, not very prominent. Surface concentrically striated. 



Length of a specimen from Gait, thirty-one millimetres : height of 

 same, twenty-one mm. : thickness, eight mm. In another example, 

 from Durham, the propoi'tions are not quite the same, the length 

 being twenty-eight mm., the height twenty-one mm. and the thickness 

 eight. 



Gait, Dr. E. Bell, 1861 : Durham, Mr. Joseph Townsend. A single, 

 nearly perfect but not very well preserved cast, with the mould in the 

 rock from which it was taken, — ^from each of these localities. 



The Anatina sinuata of Hall, which Mr. B. Billings regarded as the 

 Ame];ican type of his genus Ilionia, was described from two imperfect 

 specimens in very poor condition. Judging by the figure of the best 

 of these, the bi-oad, non-sinuated and presumably anterior end of I. 

 sinuata is much longer than the abruptly contracted and narrow pos- 

 terior end, and the beaks, which Prof. Hall says are " vertical or not 

 perceptibly inclined to either side of the shell," are consequently 

 placed at some distance behind the middle. In I. Galtensis, however, 

 the anterior and posterior ends are about equal in length, and the 

 beaks are subcentral and curve forwards. 



Ilionia (?) costulata. (N. Sp.) 

 Plate 2, fig. 5. 



Shell compressed, very gently convex, nearly equilateral, tx-ansversely 



