716 CAMBRIAN BRACHIOPODA. 



dorsal valve is usually thicker than the ventral valve, and its inner lamellae are arranged 

 more oblique to the outer layer. 



The cast of the ventral valve indicates a small apical callosity penetrated by a slender 

 pedicle tube. The main vascular sinuses are almost straight from the apex far out toward the 

 anterolateral portion of the valve; in the short valves they diverge quite rapidly, but in the 

 elongated valves they leave but a narrow space for the visceral area. From the anterior margin 

 of the apical callosity two narrow ridges extend well forward and then curve outward and inward 

 to meet about a central depression or pit that occupies the same relative position as the central 

 depression of the visceral cavity in Oholus and its subgenera. The cardinal scars have not been 

 observed. The interior of the dorsal valve is strongly concave. A slight median ridge is seen 

 in some specimens and in others it is very difficult to determine its presence except in the pos- 

 terior third of the shell, where it is often very strong. On each side of the median ridge there 

 is usually a rather deep groove in which, at about one-third the distance from the posterior to 

 the front margins of the valve, the central muscle scars occur; the cardinal scars are clearly 

 defined on each side of the median ridge near the posterior margin. 



Type. — Acrotreta proavia Matthew. 



Ohservations. — This genus is intimately related to Acrotreta and except for the presence in 

 some species of a well-marked visceral area of a type not known in Acrotreta there would be 

 difficulty in assigning more than a subgeneric value to it. Matthew [1901b, p. 303] considers 

 that the mode of occurrence of the ventral valve indicates a difference from Acrotreta, but in the 

 large collection made hj Loper the ventral valve of Acroihyra lies on the long side of the cone, 

 the aperture being up; tliis is the position the shell would take in settling down on the bottom 

 when the animal was out of the shell and the dorsal valve detached. I have found hundreds of 

 specimens of the ventral valve of Acrotreta idahoensis Walcott and A. attenuata Meek lying on 

 the upper surface of layers of limestone in situ, in just the same positions as those taken by 

 Acrothyra proavia (Matthew). 



Four species of the genus are known to me: Acrothyra proavia (Matthew), A. signata 

 Matthew, A. sera (Matthew), and A. mmor Walcott. Matthew [1902a, pp. 381-390] has described 

 a number of varieties of the two Acadian species. Jn the case of the varieties of A. proavia I 

 find that there is such a gradation of form and character between the varieties that they are of 

 little value. In A. signata the varieties prima and orta are apparently distinctive. 



After a thorough comparison of Matthew's types of Acrothyra signata sera, Acrothyra signata 

 tarda, Acrotreta papillata and varieties prima and lata, a.nd a large series of specimens from the 

 same locality and stratigraphic horizon, I have identified the four forms as Acrothyra sera. 

 There is such a gradation in size and shape of the visceral area and exterior of the ventral valve 

 that it does not appear practicable to draw lines establisliing varieties and species. The gradation 

 of characters between Matthew's Acrothyra signata sera and his Acrotreta jiapillata also practically 

 brings together Acrothyra and Acrotreta. An attempt is made on Plate LXXX to show some of 

 the gradations in the form of the visceral area (figs. 6d, 6e, 5a, 6f, 6g, 6h, and 6a). 



Matthew [1902a, p. 381] assigns importance to the stratigrapliic position of the specimens 

 he has referred to species and varieties. Tliis does not appear to hold good in the large collec- 

 tions made by Loper. Most of the varieties in form and robustness occur at the horizon of 

 Acrothyra sera and those assigned to A. proavia at its horizon. This makes it difficult to give 

 the same importance to the variations that Matthew [1902a, pp. 381-390] does in his descriptions. 

 The latter are presented in great detail and should be consulted by those interested in this genus 

 or in the Acrotretidss. 



Matthew [1901c, pp. 93-107] has made some most interesting and suggestive observations 

 on the resemblance of the more elongated forms of Acrothyra to some species of Hyolithes, sugges- 

 tions to which the student of the Brachiopoda should give careful consideration. 



