A A 



ACROTRETIDiE. 717 



ACEOTHYKA MINOR Walcott. 

 Text figure 59; Plate LXXVI, figures 4, 4a-b. 

 Acrothyra minor Walcott, 1905, Proc. U. S. Nat. Mus., vol. 28, p. 303. (Characterized as below as a new species.) 



This species differs from others referred to the genus by its broad form and very strong 

 vascular sinuses. The elevated callus between the sinuses is high and oval in outline, some- 

 what like that of Acrotreta inflata (Matthew). It maj^ be that this species 

 belongs to a different genus, but with the material available for study it 

 is referred to Acrotliyra on account of its low overhanging false area and 

 elongate visceral area. 



Formation and locality. — Middle Cambrian: (5b and 54s) a Dark blue-gray Langston 



limestone [Walcott, 1908f, p. 198], just above the Cambrian quartzitic sandstones, north side 



of Twomile Canyon, near its mouth, 2 miles (3.2 km.) southeast of Malade, Oneida County, ^"^™^, ^^T'^f™'*/™ ?™°'' 

 -J ' ' ^ ' ' - J) Walcott. A, A', Exterior 



-Laano. and side views of a ven- 



tral valve from Locality 



I Acrothyra proavia (Matthew). s*^' ^'^°"t 2 miles (3.2 



\ ^' km.) southeast of Malade, 



Plate LXXV, figure 3; Plate LXXVI, figures 3, 3a-u. . ctf No.^52061)!'''*' ^'''' 



Acrotreta proavia Matthew, 1899. Bull. Nat. Hist. Soc. New Brunswick, vol. 4, pt. 3, No. 19, p. 203, PI. Ill, figs. 2a-f. 



(Described and discussed as a new species.) 

 Acrothyra proavia prima Matthew, 1901, idem, vol. 4, pt. 4, No. 19, figs. 1-6, p. 303. (No mention of this species is 



made in the text, which is a description of the genus Acrothyra, but the variety is figured for the first time. The 



specimen represented by fig. 1 is redrawn in this monograph, PI. LXXVI, fig. 3.) 

 Acrgthyra proavia Matthew, 1902, idem, vol. 4, pt. 5, No. 20, pp. 386-388, PI. XIV, figs. 2a-g and 3a-f. (Described 



and discussed. Figs. 2a-f are copied from Matthew, 1899, PL III, figs. 2a-f.) 

 Acrothyra proavia prima Matthew, 1902, idem, p. 389, PI. XIV, figs. 4a-f. (Described. Figs. 4a-f are copied from 



Matthew, 1901b, figs. 1-6, p. 303.) 

 Acrothyra proavia crassa Matthew, 1902, idem, pp. 389-390, PL XIV, figs. 5a-c. (Described and discussed as a new 



variety.) 

 Acrothyra proavia Matthew, 1903, Geol. Survey Canada, Kept. Cambrian Rocks Cape Breton, pp. 53-55, 57, and 59. 



(Notes on orientation of shells.) 

 Acrothyra proavia prima Matthew, 1903, idem, p. 58. (Notes on orientation of shells.) 

 Acrothyra proavia prima Matthew, 1933, idem, figs. 1-6, p. 86. (Figs. 1-6 are copied from Matthew, 1901b, figs. 1-6, 



p. 303.) 

 Acrothyra proavia prima Matthew, 1903, idem, p. 93, PL II, figs. 4a-f. (Text and figures copied from Matthew, 1902a, 



p. 389, PL XIV, figs. 4a-f.) 

 Acrothyra proavia crassa Matthew, 1903, idem, p. 94, PI. II, figs. 5a-c. (Text and figures copied from Matthew, 1902a, 



pp. 389-390, PL XIV, figs. 5a-c.) 



The generic description is founded on this species and gives all of its principal characters. 

 Matthew refers to several varieties of tliis species and [1901b, p. 303, and 1902a, p. 389] names 

 two of them, prima and crassa. Loper's collection shows a great variation in the form and out- 

 line of the valves, particularly the ventral, that appears to include the varieties described by 

 Matthew. The typical form of A. jwoavia is illustrated by Plate LXXVI, figures 3a, 3b, and 

 3c; the variety prima by figure 3, and the variety crassa by figures 3e and 3f. The gradations 

 in form between the typical form and its varieties may be traced by the figures on the plate, 

 and by a study of the large series of specimens in the collections. 



The height of the posterior side or false area has been found one-third, one-half, and about 

 three-fifths of the length of the anterior slope from the apex to the margin. The wide variation 

 appears to be due very largely to compression and distortion in the sediment. 



Acrotreta gemmula Matthew occurs in association with this species, and this leads to confu- 

 sion of the dorsal valves of the two species unless care be taken to distinguish them. The A. 

 gemmula shells are rare, and the shell is thinner, which gives less strongly marked interiors, and 

 less convexity to the outer surface. The dorsal valve of Acrothyra proavia is convex, strong, 

 and deeply marked by the grooves made by the main vascular smuses. 



«5b is the type locality. 



