594 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxv. 



ACROTRETA SAGITTALIS Salter. 



Obolella sagittalis SAI.TER, Rep. Brit. Assn. Adv. Sci., for 1865, 1866, p. 285. 

 Dischia lahiosa Salter, Rep. Brit. Assn. Adv. Sci., for 1865, 1866, p. 285. 

 Oholdla sagittalis Davidson, Geol. Mag., V, 1868, p. 309, pi. xv, figs. 17-24; Brit. 

 Foss. Brach., Ill, 1871, p. 339, pi. l, figs. 1-14; Y, 1883, Pt. 2, p. 211. pi. xvi, 

 figs. 25, 26. 

 Oholella sagittalis Linnarsson, Brach. Paradoxides Beds of Sweden. Bihang till 



k. Svenska vet. Akad Handl., Ill, 1876, No. 12, p. 19, pL in, figs. 36-41. 

 Linnarssonia sagittalis Walcott, Amer. Jour. Sci., 3d ser., XXIX, 1885, pp. 115, 

 116; Tenth Ann. Rept. U. S. Geol. Sur., 1891, p. 610, pi. Lxviii, figs. 2, 

 2a-d. 

 Oholella {Linnarssonia) pretiosa D.wvsos, Trans. Roy. Soc. Canada, VII. 1889, p. 



53, figs. 26, a, b, c. 

 Linnarssonia cnf prctiosa Hall, Trans. Roy Soc. Canada, YII, 1889, p. 55. 

 Linnarssonia j^retiosa Hall and Clarke, Pal. X. Y., YIII, 1892, Pt. 1, p. 109, pi. 

 Ill, figs. 43, 44. 

 Although I have examined a large collection of this .species, both from 

 Wales and from Sweden, good exteriors of the valves have not been 

 observed. The apex of the pedicle valve almost invariabl}- remains in 

 the matrix or the shell has been removed b}^ solution. The convexity 

 of the two valves is approximately the same, except that the pedicle 

 valve is more elevated near the apex. The apex is situated a littie in 

 front of the posterior margin at the edge of the slightly defined false 

 area which slopes forward at an angle of from 70^ to 80°. The sur- 

 face of the shell is marked b\" rather coarse concentric strite and often 

 strong ridges of growth and very tine radiating striw on the interior 

 layers. The shell is built up of thin layers or lamelli« of a calcareo- 

 corneous nature. The average size of specimens from St. Davids, 

 Wales, is 2.5 mm. 



The interior of the pedicle valve is shown by numerous casts. The 

 apical callosity is usually large, as are also the cardinal scars. The vis- 

 ceral cavity is sometimes outlined in front of the callosity- and between 

 the clearlv marked main vascular sinuses. Casts of the interior of the 

 brachial valve show a remarkable range of variation in the length 

 and size of the median ridge. Within the extremes of variation 

 specific characters could be established. 



The representatives of the species from Andrarum, Sweden, are usu- 

 all}' smaller than the St. Davids shells, but otherwise appear to be 

 identical. This species has had a varied experience at the hands of 

 paleontologists. Dr. Davidson elaboratelv illustrated the casts of the 

 interiors of the valves, and I copied some of his figures when prepar- 

 ing the description of the genus Linnarssonia. When in Wales in 

 1888, I collected a quantity of material at St. Davids, and recently 

 ]Mr. Schmalensee collected a good series at Andrarum for the U. S. 

 National Mu.seum. F.rom these collections a series of figures have 

 been drawn that illustrate the appeaj-^nce of the shells as they occur 

 both in shale and in limestone. 



