75 



much longer, its margin rather narrowly rounded ; base broadly rounded, 

 most projecting in the middle, Hinge border, behind the beaks, almost 

 straight and horizontal : hinge area tlattened at a rigiit angle to the 

 sides of the valves, or a little concave. There is no defined escutcheon, 

 although the beaks are subcarinate behind, in consequence of the dis- 

 tortion which the specimen has undergone. Ligament external, large antf 

 prominent, extending along most of the length of the hinge line. Beaks 

 large, anterior, terminal, curved inwards and forwards. At the anterior 

 end, immediately under the beaks, there is a deep inflection of the 

 margin of each valve, and an ovately cordate sinus is thus formed by 

 the junction of two sunken auricles, as represented at Plate X., fig. 3a. 

 The inflection is probablj^ of the nature of a bvssal emargination, 

 though no actual opening can be detected between the valves at this 

 point. The inner faces of the sinus are perpendicular, and the auricles 

 are flattened at a right angle to the sides of the shell. Surface of the 

 test concentrically costate ; the ribs rather fine, and narrower than the 

 grooves between them. 



Greatest length, about two inches; height, in the middle, one inch and 

 nine lines ; maximum convexity, eleven lines. The greatest diameter of 

 the onl}' specimen is in the direction of a line drawn obliquely from the 

 beaks to the posterior end of the base. The shell is preserved on the 

 whole of the right valve, and on part of the left. The sculpture is not 

 xeiy well shown, but there are no traces of any radiating stria\ 



The above description is intended to apply exclusively to the curi- 

 ously distorted fossil represented on Plate X. In this specimen, (the 

 only one collected) the general direction of the compression appears to 

 have been lateral, but also a little oblique, so that the valves have been 

 partially- displaced. The beak of the right valve, accordingly, projects 

 somewhat beyond that of the left, and the left valve is quite as flat, if 

 not flatter, than the right. It is scarcely necessary to add, that in the 

 normal state the left valve is much the most convex of the two, and that 

 its beak ovei-hangs that of the right. The elongated shape of the sinus 

 under the beaks, and the blunt ridges behind them, are also obviously 

 due to the compi-ession just described. 



In 1875, Mr, G. M. Dawson collected about fifty or sixty well pre^ 

 served casts of an Aiicella, which is undoubtedly the A. PwcJv'i. of Gabb, 

 at Tatlyaco Lake,* in British Columbia. A careful study of these 



* Tatlyaco Lake ison the east branch ot the Homathca River, which empties into Bute Inlet. In some 

 maps it is spelt Tatlahco or Tatlayoco. 



