Compared 

 with other 

 species. 



Acrotreta 



bisecta 



described. 



186 



Horizon and locality. Gray shales of the assise C. 3 c 2 , at McLeod 

 brook, Boisdale, N.S. 



This little shell seems to throw light on the function of the callus in 

 Acrothyra and Acrotreta. In ordinary species of Acrotreta the strong 

 thickened ring around the foramen, within the shell, only needs to be 

 raised still further to produce a siphon. And the siphon in this species, 

 attached as it is to the dorsal side, holds the position of the callus in 

 Acrothyra. 



This must be near in age to Acrotreta gemma of Billings, than which 

 it is a little larger, but as we do not know anything of the interior of 

 Billings species (which belongs to the Arenig horizon) we do not use his- 

 name. 



ACROTRETA BISECTA, PI. XI. figs. 5a-g. 



Bull. Nat. Hist. Soc. of N. B., No. iv. p. 275, pi. xvi. figs. 2a-g. 



Shell substance thin, calcareo- corneous. Outline of the valves, oblately 

 circular. 



Ventral valve elevated conical. Height about one-quarter less than the 

 width. The umbo is about a quarter of the length of the valve from the 

 posterior margin. The valve is somewhat flattened on the posterior slope 

 at the cardinal area, which is nearly as long as half the width of the valve, 

 and has a deltidial area, bounded by distinct furrows ; elsewhere the valve 

 slopes regularly to the margin. Interior. The mould is always truncated 

 and has a somewhat convex summit ; in some examples there are traces 

 of one or two diaphragms extending across or into this part of the valve^ 

 from the anterior slope. A crescent-like ridge extends around the back 

 of the summit of the mould and down the lateral slopes. Towards the 

 front of the valve a pair of low ridges radiate toward the front of the 

 mould, but fade out at one-quarter from the anterior margin. 



The dorsal valve is most convex at the back, where the slope is nearly 

 vertical ; it has a long flattened slope to the front. Interior. The mould 

 of this species is marked by a long, deep, narrow furrow (indicating a 

 strong mesian ridge) ; this is somewhat broader in the anterior third than 

 elsewhere ; the mould also has two pairs of pits near this furrow, which 

 perhaps indicate the position of the central muscles ; the posterior adduc- 

 tors are indicated by bosses on the mould near the cardinal line, and the 

 lateral muscles by depressions near the ends of the cardinal area. Fine 

 radiating vascular lines are visible in the front half of the valve on each 

 side of the median ridge. In young valves this ridge is only two-thirds 

 of the length of that in the adult valve, the anterior third being smooth. 



* New York State Geologist's Report, 1891, Hall & Clarke, pi. i, fig. 7. 



