ACROTRETID^. 673 



The cast of the ventral valve shows that the interior of the shell has a rather strong cal- 

 losity or apical swelling penetrated by the foraminal tube, and on each side of and back of the 

 callosity near the posterior margin the cardinal muscle scars, which usually take the form 

 of a projecting boss or tubercle corresponding to a depression in the shell in which the trans- 

 median and middle lateral muscles were probably attached. In front of the apical callosity in 

 A. argenta Walcott there are two trapezoidal areas con-espondhag to similar areas in Oholella 

 and Oholus, in which the central, outside, and middle lateral muscles were attached. The 

 position and size of the areas are shown by Plate LXVII, figures 5a and 5b. The grooves of 

 the main vascular sinuses pass around the apical swelling and extend forward, diverging toward 

 the anterolateral margins of the shell. 



The interior of the dorsal valve is almost invariably marked by a long, well-defined median 

 ridge and a pair of strong cardinal tubercles near the margin of tlie area corresponding in 

 position to the cardinal tubercles of the ventral valve. Smaller tubercles occur in advance 

 of the posterior tubercles, one on each side of the median ridge; they are often replaced by elon- 

 gate oval scars that correspond to the central .scars of the dorsal valve of Odolus: the anterior 

 lateral muscle scars are too minute to be clearly located, but they were undoubtedly close to 

 the median line a little in advance of the central scars. The cardinal tubercles of both valves 

 often have scars on them indicating the attachment of muscles. 



Type. — Acrotreta suhconica Kutorga. 



Observations. — Of the type of the genus, A. suhconica Kutorga, only the external shell is 

 known. In 1884 [1884b, PL I, figs, lb and Id] I illustrated the interior of both valves of "A. 

 gemma" (see pp. 30-31), showing the area, median ridge, "cardinal tubercles," and central muscle 

 tubercles of the dorsal valve, and the apical swelling and cast of the main vascular canals in the 

 ventral valve. Matthew [1886, PL V, figs. 13 and 13c] illustrated the interior of A. haileyi, 

 showing the cast of a small tubercle on each side of the apical swelling. Hall and Clarke [1892c, 

 p. 102] stated that they could not see the cast of these tubercles on the original specimen, and I 

 have not been able to find them on any of the type specimens of A. haileyi received from Mat- 

 thew. It frequently occurs that the casts of the short deep channel of the main vascular 

 sinuses give rise to tubercles a little in advance of and close to the apical callosity (PL LXX, fig. 

 If), or it may be that the cast of the visceral cavity will take the form of tubercles in front of 

 the cast of the apical callosity (PL LXIV, fig. 2c). 



The intimate relations existing between Acrotreta and Linnarssonia became more and more 

 apparent with the collection of better material representing the two species referred to the two 

 genera. Girty, when selecting specimens for illustration, called my attention to the strong 

 resemblance between them and suggested that Linnarssonia was a synonym of Acrotreta. In 

 all essential characters this is true, and Linnarssonia, if retained at all, must be as a subgenus to 

 include the depressed forms of Acrotreta, intermediate between the typical elevated species A. 

 suhconica Kutorga and A. gemma Billings and the nearly flattened shells of Acrothele. In view, 

 however, of the close similarity of the interiors of the valves of Acrotreta scTimalenseei Walcott, 

 A. curvata Walcott, and A. kutorgai Walcott to "Linnarssonia transversa" (see Acrotreta sagitta- 

 lis transversa (Hartt)), "'L." sagittalis (Salter), and "L." misera (Billings) there remains only 

 the external form of the pedicle valve to distinguish the two genera; this latter character is 

 not of generic importance in the presence of the gradation in elevation and outline between 

 A. suhconica Kutorga and A. sagittalis transversa (Hartt), shown by A. idahoensis Walcott, A. 

 scJimalenseei Walcott, A. microscopica (Shumard), A. pyxidicula White, and A. sagittalis (Salter). 



When studying the specimens of Acrotreta at hand in 1884 I believed that A. gemma 

 Billings, of Newfoundland, included the western forms described by Meek [1873, p. 463] and 

 White [1877, p. 53]. This conclusion was retained until the study of a large series from each of 

 the typical localities convinced me that there were a number of species and varieties grouped 

 under A. gemma. The correct reference of these species and varieties is indicated in the table, 

 pages 30-31, and taken up in detail in the synonymy of each of the different species. 

 62667°— VOL 51, ft 1—12 43 



