1916] Packard: Mesozoic and Cenozoic Mactrinae 291 



margin of the plate. Pallial sinus deep, rounded, not extending 

 in front of the vertical of the beaks. The dimensions of the type 

 as inferred from the original cut, are : length, 45 mm., height, 

 35 mm. 



The type of this species was collected by Townsend and de- 

 scribed by Conrad, together with other fossil material from a 

 locality on the Columbia River, near Astoria. Gabb confused this 

 variable species with Spisula planulata. The species was recognized 

 by Arnold in the Lower Etchegoin formation of the Coalinga Dis- 

 trict, California. Arnold's figure of the form differs considerably 

 from that of the type and also from that given by Dall from the 

 Empire formation of Coos Bay, Oregon. 



These three forms represent but three different shapes assumed 

 by this species. In some collecting localities these forms occur 

 together, while in others one form or another predominates. There 

 appears to be no general type found in the San Pablo group that 

 is not equally well represented by specimens from the Merced. A 

 comparison of the hinges of these different specimens shows a 

 remarkable uniformity as regards the dental armature. 



Spisula albaria is closely related to S. albaria var. ramonensis, 

 from which it differs in being typically more elongate, with a nar- 

 rower hinge plate and shorter laterals, which are more intimately 

 connected with the cardinal arm. It differs from S. selbyensis in 

 being more nearly equilateral, in its wider hinge plate, less com- 

 pressed left cardinal, shorter laminae and in the less intimate con- 

 nection between the anterior ventral lamina and the cardinal arm. 



The type of S. precursor was obtained from the Empire Forma- 

 tion of Coos Bay, Oregon. The outline of this species might easily 

 fall within the range of the shape-variation of 8. albaria. It is 

 therefore considered as probably representing that species. 



Occurrence. — Pliocene : Merced group. Miocene : San Pablo 

 group. 



SPISULA ALBARIA (Conrad) var. RAMONENSIS, n. var. 

 Plate 23, figure 5; plate 25, figures 1 and 2 



Shell medium-sized, inequilateral, somewhat compressed ; height 

 nearly equal to that of the length ; umbones rather prominent, 

 sharply pointed; shell smooth except for an occasional heavy incre- 

 mental line of growth ; anterior dorsal area excavated below the 



