its narrow umbonal ridge to the ventral valve, higher cardinal area, and 

 by the longitudinal pits that enclose the umbonal ridge. It differs from 

 .A. gemmula of the Protolenus faunaj (see PI. III., tigs 4 a-d) in the sharp 

 umbonal ridge of the interior of the ventral, and by the strong lateral 

 ridges of the inside of the dorsal valve. From Acrothyra proavia of the 

 Etcheminian fauna (see PL II., figs. 2 a-g] it is distinguished by the 

 more central apex of the ventral valve, and by the prominent ridges of the 

 interior of the dorsal valve, as well as by its larger size. "From A. gemma, 

 Bill. (Walcott)U it is distinct by its smaller size, less proportionate height 

 of the ventral valve, its narrow umbonal boss and its more obscure 

 cardinal area, also by a difference in the internal markings of the dorsal 

 ' valve. 



ACROTRETA sipo, PL XVIII. figs. 1 and 2. 



Bull. Nat. Hist. Soc. of N.B., No. iv. p. 406, pi. xviii figs, 1 and 2. 



Acrotreta sipo 



A small species with somewhat overhanging umbo. described. 



Ventral valves nearly as high as long. Umbo projecting behind the 

 cardinal line, somewhat bluntly pointed, (some valves are trumpet shaped 

 toward the margin) and a little broader than long. Interior. The fora- 

 men passes outward through a short siphon which is attached to the dor- 

 sal side of the valve ; on each side of it are traces of lateral septa ; in 

 front of it is the faint impression of a callus which extends one-third of 

 the distance to the anterior margin. 



The dorsal valve is transversely oval, and arched from hinge to front, 

 more strongly toward*the hinge ; the lateral edges are flattened, especially 

 toward the hinge. Interior. This shows traces of scars of lateral (?) 

 muscles on each side of the umbo, and of a pair of central muscles near 

 the middle of the valve. A distinct, though low, median septum crosses 

 the valve nearly to the front margin. 



Sculpture. No concentric strise were observed on this species, but the 

 surface of the valves is minutely granulated. 



Size. The largest ventral observed had a size at the orifice of 3 x 3 

 mm., and others a height of 2 mm. A dorsal was 2| x 3 mm. ; height 

 f of a mm. 



The siphon is seldom preserved. 



Three quarters of the ventral valves collected stand vertically in the 

 mud in which they were entombed. 



J Trans. New. York Acad. Sci. xiv, p. 126, pi. v, figs. 5 a-d. 

 Bull. Nat. Hist. Soc. N.B., vol. iv, p. 203, pi. iii, figs. 2 a-f. 

 IT Bull. U.S. Geol. Surv. 30, p. 98, pi. viii, figs. 1, la-6. 



