NO. 1299. CAMBRIAN BRACHIOPODA-WALCOTT. 579 



of the brachial valve is almost invariably marked by a long, well- 

 defined median ridge and a pair of strong- cardinal' tubercles near 

 the margin of the area, corresponding in position to the cardinal 

 tubercles of the pedicle valve. Smaller tubercles occur in advance of 

 the posterior tubercles, one on each side of the median ridge. They 

 are often replaced by elongate oval scars that correspond to the 

 central scars of the dorsal valve of Oholus. The cardinal tu])ercles 

 of both valves often have scars on them, indicating the attachment of 

 the muscles. 



Ohservatmis.—Oi the type of the genus, .1. siibconica, only the 

 external shell is known. In 1884 I illustrated the interior of the two 

 valves of A. gemma,^ showing the area of the median ridge, the ^'car- 

 dinal tubercles'' and the central muscle tubercles of the dorsal valve, 

 and the apical swelling and cast of the main vascular canals of the 

 ventral valve. In 1886 Mr. G. F. Matthew illustrated the interior of 

 A. haileyl,'' showing the cast of a small tubercle on each side of the 

 apical swelling. Messrs. Hall and Clarke write that they could not 

 see the cast of these tubercles on the original specimen, ^ and I have 

 been unable to find them on the specimens that Dr. Matthew very 

 kindly sent me as the types. The intimate relations existing between 

 Acrotreta and Linnarsso7iia became more and more apparent as l)etter 

 material was obtained of the two species referred to the two genera. 

 Dr. G. H. Girty, when selecting specimens for illustration, called my 

 attention to the strong resemblance between them, and suggested that 

 Lbrnarssonia was a synonym of Acrotreta. In all essential characters 

 this is true, and Lmnarmmla, if retained at all, nmst be as a subgenus 

 to include the depressed forms of Acrotreta intermediate between the 

 typical elevated species A. mlconica, A. gemma, and the nearly flat- 

 tened shells of Acrothele. In view, however, of the close similarity 

 of the interiors of the valves of Acrotreta sckmalensei, A. carvata, A. 

 I'utorgai, and Lhouumonla transversa, L. sagittalis, and Z. mdera 

 there remains only the external form of the pedicle valve to distin- 

 guish the two genera; this latter character is not of sufficient impor- 

 tance to base a subgenus on, in the presence of the series of gradation 

 of elevation and outline between A. sulconica and A. transversa shown 

 by A. idahoensis, A. schmalensei, A. microscopica, A. pyxidicula. A. 

 sag itt alls, and A. transversa. 



When studying the specimens of Acr<)trda at hand in 188-4 I believed 

 that A. gemma of Newfoundland included the western forms described 

 by Messrs. Meek and White. This conclusion was retained until the 

 study of a large series from each of the typical localities convinced me 



'Mong. U. S. Geol. Sur., VIII, pi. i, figs, lb, Id. 

 ^Trans. Roy. Soc. Canada, III, pi. v, figs. 18, 13c. 

 ^Pal. New York, VIII, Pt. 1, p. 102. 



