92 



Resemblance 

 to Hyolithes. 



Characterizes 

 the Upper 

 Etcheminian 

 Fauna as A. 

 signata does 

 the Lower. 



Additional 

 characters of 

 the species. 



might be mistaken for those of a minute Lingulella. As they occur 

 scattered over the layers of the shale they also strongly recall the ordi- 

 nary aspect of the conical teeth of fishes, brilliant with black enamel. 



No described species of Acrotreta is as small as the more abundant 

 valves of this species, though A. gemmula of the Protolenus fauna approa- 

 ches it in that respect.* 



Sir William Dawson has called attention to the resemblance in struc- 

 ture between the shells of Hyolithidse and the Brachiopoda, and has com- 

 pared the ventral valve of a Brachiopod to the tube of a Hyolithes. Had 

 Sir William been acquainted with this species ho would have found it a 

 good example for comparison. This will be seen if the ventral valve be 

 so oriented as to make the. areal side correspond to the ventral side of a 

 tube of Hyolithes. The dorsal valve with its round form and excentric 

 umbo, with radiating lines, also resembles the operculum of a Hyolithes. 

 A detailed comparison of this species with certain Hyolitidse has been 

 made in an article contributed to the Hoyal Society of Canada (Trans. 

 New Ser. Vol. VII., Sec. IV. p. 93). 



A study of layers of the shales of the horizon E. 3 /-., studded with the 

 valves of this species, failed to reveal any ventral valves, showing clearly 

 a thickened callus. For the relationship of this species we have therefore 

 to depend on the forms prima and crassa, both of which possess a narrow 

 callus. These show that these three forms are of the ;-aine genus as A. 

 signata, but of a different species, and reveal a series in the upper Etche- 

 minian Fauna parallel to the Siynati of the lower fauna ; they are distin- 

 guished from the latter by their narrow visceral callus. The absence of 

 a thickened callus in the typical form of A. proavia would seem to show 

 that the pedicle in this form was slender and weak, and from the fact that 

 this shell, above all its fellows, shows a perfect orientation in one direc- 

 tion, as imbedded in the shale, there is a presumption that the pedicle 

 was also long, enabling the animal to swing in the currents of the sea in 

 which it lived. Often the ventral has an even slope along the back, 

 but many old valves, especially long ones, show from two to three 

 heavy concentric ridges, marking stages of the growth of the shells. 



Interior. The ventral valve of this species has a quite small tubercle 

 in front of the foramen. Two-fifths from the apex of the ventral valve 

 there is a shallow depression on the interior surface, which, by analogy 

 with mut. prima should mark the position of the central muscle scars. 

 On each side of the shallow depression a shallow groove runs forward 

 toward the front of the valve. Some examples show a median and two 

 lateral septa in front of the shallow depression. Faint ridges, running 



* Trans. Roy. Soc. Can. Vol. xi, p. 87, pi. xvi, figs 2 a to d. 



