586 PROCEEDINGS OF THE NATIONAL MUSEUM. 



specimens, and fainth' in others. Two shells have the apex broken oft' 

 so as to expose the cast of a minute apical callosity and a small portion 

 of the side of the cast of the main vascular sinus, 



A. gemma belongs to the ^4. svhconiea group of the genus, having- a 

 high pedicle valve and distinct and broad false area. It difters from 

 A. .'^nhconica in having a less elevated pedicle valve and in the apex 

 being in front of the posterior margin instead of extending over it. 

 With the forms from the Rocky Mountains that I identified with it there 

 are no strong specific relations, ^l. atUnuata has a high pedicle valve, 

 but the false area is practicall}" aljsent, a narrow deep sulcus taking its 

 j)lace. A. idahoensis alta has the elevation and false area, but differs 

 in the details of the area and the outline of the cross section of the 

 pedicle valve. It was this form that led me to consider that A. gemma 

 occurred in Nevada, and with the slight knowledge that I then had of 

 the genus and species a wide range of form was given to ^i. gemma. 

 As far as now known to me. the species is restricted to the type 

 locality. 



Formation and locality. — Lower Ordovician, limestone, North 

 Table Head and Pistolet Bay; limestone point -1 miles northeast of 

 Portland Creek. Newfoundland. 



ACROTRETA GRACIA, new species. 



Shell small, slightly transverse; apex of pedicle valve about one- 

 third the length of the shell from the posterior margin; height about 

 two-thirds the length of the shell; false area defined by the sharp 

 rounding of the cardinal slopes and the transverse posterior margin, 

 which is indented by the strong, broad, median groove extending from 

 the margin to the apex, where it almost comes in contact with the 

 minute apical foramen. Brachial valve moderatel}' convex, with a 

 nearly straight or gently curved posterior margin; beak minute, mar- 

 ginal. Surface of shell marked by fine concentric strias and lines of 

 growth. Shell strong, and built up of thin layers or lamellaj that 

 over the central and anterior portions are more or less oblique to the 

 outer surface layer. 



The cast of the pedicle valve indicates a small but \qy\ clearly 

 marked apical callosity; rather small cardinal scars and main vascular 

 sinuses. The cast of the interior of the lirachial valve shows a broad, 

 low, median ridge extending to about the anterior third of the length 

 of the valve; small cardinal scars and small main vascular sinuses that 

 arch inward after passing the central scars, and then outward. The 

 urea is very narrow, and broken midway by a rather wide, slightlj^ 

 arching false deltidium. 



This species is distinguished by the strong shell, broad median ridge 

 of the brachial valve, and deep median groove of the false area of the 

 I)edicle valve. The light color of the shell may be owing to the char- 



