ACROTRETID^. " 677 



decision as to whether the umbo or the apex is the most elevated part of the valve. False 

 area defined by the incurving of the cardinal slopes so as to form a somewhat flattened, trian- 

 gular space that is divided midway by a rather strong vertical furrow. Pedicle aperture 

 rather large and opening obliquely backward. The dorsal valve is gently convex, with a 

 minute marginal beak. 



Surface of shell marked by fine concentric strise and lines of growth, and possibly by fine 

 radiating striae. I fail to find any traces of the radiating striae on specimens that have not 

 been distorted by pressure, and it looks as though all radiating strise and lines are the result 

 of lateral compression of the shell. The average length is about 3.5 mm.; width, 4 mm. (Dis- 

 torted shells have a length of 4 mm.; width, 3 mm.) 



The cast of the interior of the ventral valve shows a strong apical callosity, a fair-sized 

 pedicle opening, large cardinal scars, and a small visceral area in front of the apical callosity. 

 Matthew [1886, p. 36, PL V, fig. 13c] describes and shows on his illustration two "minute 

 muscle scars" close to the "umbonal tubercle" and on each side of the parallel strise. On 

 one specimen I find what may have led Matthew to consider there were such scars, but on 

 many others quite as well preserved there are no such indications, and they are not known 

 in any other species of the genus. Hall and Clarke examined Matthew's type material, and 

 stated [1892c, p. 102] that with the original material before them the}'' were unable "to dis- 

 tinguish anything more than the central callosity." The cast of the interior of the dorsal 

 valve shows a strong median ridge of variable length and size, large cardinal scars, and well- 

 defined central scars. 



Observations. — This species belongs with the series represented by A. sagittalis (Salter) 

 and its varieties, and if uncompressed, nondistorted, well-preserved specimens could be obtained, 

 it is quite jjossible that it would prove to be identical with A. sagittalis transversa (Hartt). 

 Matthew [1886, p. 37] thinks it has a thinner shell, but that is not probable, as the apical 

 callosity and cardinal scars of the ventral valve and the median ridge, cardinal and central 

 scars of the dorsal valve all indicate a shell quite as thick as that of A. sagittalis and its varieties. 



Doctor Matthew kindly sent me the type and study material of tliis species from his private 

 collection. 



Formation and locality. — Middle Cambrian: (300) Shaly sandstones, possibly of Divmon Id of Matthew, on 

 Long Reach, Kings County; and (301) doubtfully in sandstones of Division lb2 of the " Protolenus zone" [Matthew 

 1895a, p. 108] Hanford Brook, St. John County; both [Matthew, 1886, p. 37] in New Brunswick, Canada. 



ACEOTEETA BELLATTJLA Walcott. 



Plate LXXVIII, figures 4, 4a-d. 



Acrotreta hellatiila Walcott, 1908, Smithsonian Misc. Coll., vol. 53, No. 3, pp. 93-94, PI. IX, figs. 4, 4a-b. (Described 

 and discussed as below as a new species. Figs. 4, 4a-b are copied in this monograph, PL LXXVIII, figs. 4, 

 4b, and 4d, respectively.) 



General outline subcircular to transversely broad oval, with the posterior margin of the 

 ventral valve nearly straight beneath the false area. The ventral valve is' moderately convex, 

 with the apex a little in front of the posterior margin. False area defined by the incurving 

 of the cardinal slopes so as to form a somewhat fiattened triangular space that is divided mid- 

 way by a narrow and rather shallow, vertical furrow; pedicle aperture of medium size and 

 opening slightly backward. Dorsal valve nearly as convex as the ventral, except that it curves 

 down in the posterior portion to a minute marginal beak. 



Surface of the shell marked by fine, concentric strise and lines of growth that show marked 

 variations in their sharpness on different specimens, the older shells having a dark, dull sur- 

 face giving the effect of a thin film over the strise; a few traces of radiating strise are shown on 

 some shells. 



The average length of the larger shells is about 1.75 mm.; width, 2 mm. 



The interior of the ventral valve shows a small apical callosity, minute pedicle tube, and 

 vascular sinuses originating a little back of the apical callosity; a cast shows that there were 



