36 G. F. MATTHEW : ILLUSTRATIONS OF 



L. transversa ; the aj^ical, iuteriial tubercle is smaller, the muscular scars at the hinge Hue 

 are also smaller, aud the ridges that diverge from behind the apical tubercle and extend 

 toward the front of the shell are less conspicuous. 



Sculpture. Outer surface of the shell smooth, with concentric undulations of growth ; 

 under the ieus fine concentric stria^ are visible, aud a few faint radiating striœ. 



The shell substance in this species is very thin ; frequently only the part of the shell 

 which contains the vital organs is black and shining from the presence of phosphatic 

 matter. Notwithstanding the existence of a small hinge area in this species it is so near 

 the type of Linnarssoiiia in other respects that it seems to find its place in that genus 

 rather than in Acrotreta. 



Through the kindness of Mr. J. F. Whiteaves, the author has been able to examine 

 the type of Obolella (?) misera, of Billings, from the Cambrian rocks of Newfoundland. Only 

 the dorsal valve of this species is preserved, and it agrees as to size, form, and surface 

 markings with our species, but the internal characters are not exhibited. Mr. Billings 

 describes the ventral valve as depressed conical, and in this respect his species would 

 agree with the little shell now under consideration, which, owing to its false hinge area, 

 has a somewhat conical valve. But not having seen the A^eutral valve of O. (?) misera or 

 the internal markings of either valve of the Newfoundland shell, the author cannot be 

 sure of its identity with the Acadian species. 



Length, 2\ mm. Width, 3 mm. 



Horizon and Locality. In the fine dark shales of Div. l.d, at Porter's Brook, St. Martin's, 

 N.B. 



ACROTRETA, Kutorga. 



Acrotreta Baileyi, n. sp. (Plate V. Figs. 13, 18a, b and c.) 



Transversely oval orbicular. Greatest width somewhat in front of the middle of the 

 shell. The margin is nearly straight on the hinge line, which is less than half of the 

 transverse diameter. 



The dorsal valve is moderately convex, strongly arched iipward from hinge to front, 

 but less arched transversely. The interior of the valve has a short mesian ridge, slightly 

 expanded at the extremity, which is more than half way from the hinge line to the front 

 margin. From this ridge two others of less than half its length diverge, arching outward 

 from near the hinge line. Two subtriangular muscular scars lie between these short 

 ridges and the posterior margin of the A'alve, near the hinge line ; two other fainter tri- 

 angular muscular scars lie between the two diverging ridges and the mesian ridge. 



The ventral valve is conical with a moderately elevated umbo, which is less than one 

 third of the longitudinal diameter from the hinge line ; the valve is thickened at the 

 umbo, where a prominent boss or tubercle appears within the shell. The hinge area is 

 triangular, has a sharp vertical groove in the middle, aud is crossed by a few transverse 

 striœ. The umbonal tubercle is the most prominent feature within the shell, aud from 

 its front several parallel stria? extend to the anterior margin ; two minute muscular scars 

 are visible in front of the tubercle, close to it, on each side of the parallel striti?. The 

 interior of this valve (as preserved in the shale) has several concentric ridges, strongest at 

 the anterior half of the valve. 



