686 CAMBRIAN BRACHIOPODA. 



AcROTRETA GEMMXiLA Matthew. 

 Plate LXVI, figures 3, 3a-c; Plate LXXVII, figures 5, 5a-f. 



Acrotreta gemmula Matthew, 1894, Trans. Eoy. Soc. Canada for 1893, 1st ser., vol. 11, sec. 4, No. 8, pp. 87-88, PI. XVI, 



figa. 2a-d. (Described as a new species. The specimens represented by figs. 2a and 2b are redra-wn in this 



monograph, PI. LXVI, figs. 3a and 3b-c, respectively.) 

 Acrotreta gennmula Matthew, 1895, Trans. New York Acad. Sci. for 1894-5, vol. 14, p. 126, PI. V, figs. 5a-d. (Changes 



stratigraphic range. Copies figures from preceding reference.) 

 Acrotreta gemmula Matthew, 1903, Geol. Survey Canada, Kept. Cambrian Rocks Cape Breton, p. 97, PL III, figs. 



4a-d. (Mentioned. Figures copied from Matthew, 1894, PL XVI, figs. 2c, 2d, 2a, and 2b, respectively.) 



Shell small, nearly circular m outlme, with the posterior margin slightly straightened. 

 Height of ventral valve above the posterior margin about one-half the length, of the valve. 

 Apex slightly incurved over the false area, which varies from nearly vertical to an appreciable 

 forward slope from the margin. False area with rounded lateral margins and a rather strong 

 median groove that termiaates in a sulcus at the margin and begins at a minute foraminal 

 aperture just beneath the apex. The position of the pedicle aperture causes it to open almost 

 directly backward. The dorsal valve is moderately and rather uniformly convex. Surface 

 of shell marked by fine, concentric striae and lines of growth, and toward the outer margin 

 by very fine, undulating, concentric strise that give the surface, when examined by a strong 

 lens, the fretted appearance shown on a larger scale on Oholus ( Westonia) ella (Hall and Whit- 

 field) and a few species of Acrotreta. The size of the shell is about that of Acrotreta convexa 

 Walcott, 1.5 to 2 mm. in diameter. The shell appears to have been rather thin, as it is indented 

 by the pressure of the fine grains of sand of the matrix, and often bends rather than breaks when 

 compressed. It is built up of very thin layers or lamellae that in the dorsal valve form a 

 thickened rim. 



The cast of the interior of the ventral valve indicates a small but distinctly marked apical 

 callosity penetrated by a rather strong pedicle tube that is directed backward at an angle of 

 about 45°. A trace of the main vascular sinuses is shown at the anterolateral edges of the 

 apical callosity; the casts of the cardinal scars are rather large for so small a shell. The interior 

 of the dorsal valve is marked by a rather strong median ridge, large cardinal scars, and small 

 central scars. One of the strongly marked characters is the strong outer rim. 



This is a very clearly defined species and the oldest of the Acadian representatives of the 

 genus, occurring, as it does, just below the " Protolenus fauna." The position of the pedicle 

 aperture is similar to that of many species of Acrothele and is unusual for the genus, and may 

 be of generic or subgeneric importance. 



In the collections made by Loper on Dugald Brook this species occurs with O&oZus (Palscoiolus) 

 hretonensis (Matthew) in beds beneath Acrothyra proavia (Matthew) . He collected a fine series of 

 this species at the same horizon from which Matthew obtained the types of his "Acrotreta papillata" 

 (see Acrothyra sera (Matthew), p. 718). I find that when the outer surface of the shell is well 

 preserved it has a slightly fretted or pitted appearance on specimens from Hanford Brook and 

 also Dugald Brook. Those from Dugald Brook show radiating lines when the thin outer layer 

 of the shell is exfoliated. In some layers of rock all the shells are finely pitted, evidently by 

 the fine grains of quartz sand in the matrix being pressed against the shell; in other layers the 

 shells are identical in surface markings and form with those of A. gemmula from Hanford 

 Brook. 



The interiors of the dorsal valve are not sufficiently well preserved for comparison, but in 

 the ventral valve they are essentially the same. In each locality there is considerable variation 

 in the form and size of the apical callosity. 



Formation and locality. — Middle Cambrian: (301a [MattJiew, 1894, p. 88]) Fine-grained sandstones below the 

 Protolenus-hearing shales and sandstones in Division IbS of Matthew on Hanford Brook; and (301 [Matthew, 1895a, 

 p. 126]) sandstones of Division lb2 of the "Protolenus zone " of Matthew on Hanford Brook; both in St. John County, 

 New Brunswick. 



(lOp) Sandstone just below the waterfall in Division E2b of Matthew's [1903, p. 21] Etcheminian, Dugald 

 Brook, Indian River; and (lOp") sandstone on the small brook on the hill between the bridge over Indian River and 

 McPhees Brook; both in eastern Cape Breton, Nova Scotia, Canada. 



