90 



of the valve, where the curve becomes abrupt. Interior A callus about 

 three times as long as its width in front, extending from the beak one- 

 third of the length of the valve, sometimes there is an apophysis in front 

 of it, of equal width, sometimes an apparent extension of the callus, with 

 a median ridge dividing it lengthwise. The callus usually has a transverse 

 raised thread towards the posterior end, and sometimes another near the 

 front. A faint, narrowly triangular hollow, divides the callus from the 

 impression of the lateral muscles. 



The dorsal valve is oblately orbicular in form, with inconspicuous urubo. 

 The valve is somewhat depressed in the middle and toward the front. 

 Interior This part of the valve exhibits a medium septum in the poste- 

 rior quarter of the valve, and behind it two lateral septa, that fork from 

 near the umbo ; the place of the lateral muscles is faintly marked. 



Sculpture. Of fine concentric ridges, as with other fcrms of this 

 species. 



Size. Length of ventral, 2 mm.; width, If mm.; height, 1 mm. to 2 

 mm. Dorsal length, If mm.; width, 2 mm.; height, about mm. 



Horizon and locality. Fine, greenish-grey calciferous sandstone of E. 

 2 c. at Dugald brook, Escasonie. Not rare. 



This mutation shows a change in the direction of A. proavia of a 

 higher horizon described below. 



The ventral valves lie on their sides on the layers of the rock, but some 

 dorsals are on edge. 



ACROTHYRA PROAVIA. PI. II, figs. 2 a-g and 3 a-f. 



Acrotreta proavia, n. sp. Nat. Hist. Soc. N. B. Bull., Vol. iv, p, 203, 

 pi. iii, figs. 2 a to/. ; ibid, p. 386, pi, xiv, figs 2 a-g, and 3, a-J. 



Acrothyra Shell-substance calcareo-corneous. The thin outer crust sometimes 



wanting from corrosion, abrasion or absorption. 



Ventral valve oblique conical, with a prolonged beak. Cardinal area 

 narrow, as is also the pseudo deltidium ; in the pseudo deltidiuin near the 

 apex is a small oval tubercle, between which and the apex, the foramen 

 is supposed to be situated. The valves slope evenly down from the apex 

 to the anterior and lateral margins. No good examples of the interior of 

 this valve have been obtained ; imperfect ones show two vascular lines 

 enclosing a narrow visceral callus, and extending as far dovrn from the 

 apex on one side, as the hinge area does on the other ; the front margin 

 exhibits on the interior a row of about ten radiating vascular ridges. 



