ACEOTKETID^. 687 



I ACROTEETA GBACIA Walcott 



Plate LXVI, figures 5, 5a-d. 



Acrotreta gracia Walcott, 1902, Proc. U. S. Nat. Mus., vol. 25, pp. 586-587. (Described and discussed essentially 

 as below as a new species.) 



Shell small, slightly transverse; apex of ventral valve about one-third the length of the 

 shell from the posterior margin; height about two-thirds the length of the shell; false area 

 defined by the sharp rounding of the cardinal slopes and the transverse posterior margin, which 

 is indented by the strong, broad, median groove extending from the margin to the apex, where 

 it almost comes in contact with the minute apical pedicle opening. Dorsal valve moderately 

 convex mth a nearly straight or gently curved posterior margin; beak minute, marginal. Sur- 

 face of shell marked by fine concentric striae and lines of growth. Shell strong and built up 

 of thin layers or lamellae that over the central and anterior portions are more or less obHque 

 to the outer surface layer. 



The cast of the ventral valve indicates a small but very clearly marked apical callosity; 

 rather small cardinal scars and main vascular sinuses. The false area is narrow and broken 

 midway by a rather wide, slightly arching false deltidium. The cast of the interior of the 

 dorsal valve shows a broad, low, median ridge extending to about the anterior third of the 

 length of the valve, large cardinal muscle scars, and small main vascular sinuses that arch 

 inward after passing the central scars, and then outward. 



Observations. — This species is distinguished by the strong shell, broad median ridge of 

 the dorsal valve, and deep median groove of the false area of the ventral valve. The light 

 color of the shell may be owing to the character of the limestone in which it is preserved, or 

 it may be that it is a little more calcareous than other species of the genus. 



The Acrotreta which Matthew [1897b, p. 169] describes and illustrates from the Hastings 

 Cove locaHty {" Linnarssonia helti magna") is characterized by a narrow median ridge in the 

 dorsal valve and other characters not present in A. gracia. In the Paradoxides zone on Han- 

 ford Brook I found numerous examples of a species of Acrotreta that appears to be identical 

 with the form described by Matthew [1897b, p. 169] and have referred it as a variety (magna) 

 of Acrotreta sagittalis (Salter) (p. 706). 



Formation and locality. — Middle Cambrian: (2s) Limestone in upper part of Paradoxides zone, Hastings 

 Cove [Matthew, 1898b, p. 38], on Kennebecasis Bay, 0.5 mile (0.8 km.) northeast of Torrybum, on the Intercolonial 

 Railway, northeast of St. John, St. John County, New Brunswick. 



Acrotreta idahoensis Walcott. 

 Plate LXV, figures 1, la-i; Plate LXVIII, figures 2, 2a-g. 



Acrotreta attenuata Meek (in part), 1873, Sixth Ann. Rept. U. S. Geol. and Geog. Survey Terr, for 1872, p. 463, foot- 

 note. (Among the specimens which Meek listed as A. subconica and for which he proposed the name A. attenuata 

 in a footnote are specimens which are now referred to A. idahoensis. See also A. attenuata and A. attenuata var.) 



Not Acrotreta subconica Kutorga, 1848, Verhandl. Russ.-kais. min. Gesell. St. Petersburg for 1847, No. 12, p. 275. 

 (Referred in this monograph to Acrotreta subconica.) 



Acrotreta idahoensis Walcott, 1902, Proc. U. S. Nat. Mus., vol. 25, p. 587. (Described essentially as below as a new 

 species.) 



The general outline, form, and convexity of the two valves are so fully shown by the illus- 

 trations that detailed descriptions mil not be given. The material for illustrating the interior 

 of the ventral valve is limited, but one cast shows the position of the base of the cast of the 

 foraminal tube, and the large, main vascular sinuses (PI. LXV, fig. Id). An interior of a dorsal 

 valve and the accompanying cast (PL LXV, figs. If, Ig) present characters rarely seen in this 

 genus. The vascular canals arch out beyond the central muscle scars, and then bend in toward 

 the median ridge, and again obliquely outward to form the outer limit of a smooth, polished, 

 lanceolate-shaped surface that extends obliquely outward from the median ridge. The line 

 that extends from in front of the cardinal scars forward, subparallel to the margin of the shell, 

 and then bends abruptly in and forward is apparently the outer boundary of a very broad. 



