854 PR F. R. C. REED ON THE 



closely placed vascular sinuses extending nearly three-fourths of the shell and then 

 curving outwards ; lateral portions of valve covered with sinuous radiating fine 

 vascular markings, dividing somewhat irregularly; margin of shell showing ribs. 



The internal characters of this species mark it off Irom all British forms, though 

 externally it much resembles O. testudinaria, auct., as employed in a wide sense. 

 The grooves on each side of the cardinal process forming a tripartite mark (which 

 Davidson showed in his fig. 18a) are a feature of importance, and the development 

 of the vascular sinuses is unusually distinct, and in the pedicle-valve somewhat 

 recalls O. duftonensis. I am extremely doubtful if it should be placed in the group 

 Dalmanella, sens. str. 



Orthis (Dalmanella) testudinaria, Dalman, var. nov. gracilis. 

 (Plate X, figs. 4-6.) 



Shell subcircular, unequally biconvex to plano-convex ; hinge-line about two- 

 thirds the width of shell ; cardinal angles rounded or very obtuse. 



Pedicle- valve gently and uniformly convex, not carinate ; beak elevated above 

 hinge-line, slightly incurved ; hinge-area rather large, steeply inclined to plane of 

 valve ; delthyrium large. Brachial valve very slightly convex, more or less flattened, 

 with small inconspicuous beak, weak median depression, and small hinge-area. Surface 

 of valves ornamented with numerous fine subangular ribs somewhat irregular in size, 

 numbering about 100 on margin, curving back on each side to cardinal angles and 

 decreasing in size, increasing rather irregularly in number by subequal division into 

 two at one-fourth to one-third their length, and again sometimes into two smaller 

 ones close to margin ; interspaces rounded, narrower than ribs ; fine concentric 

 lineation over ribs and interspaces, becoming coarser and imbricated near margin. 



Dimensions. — Length, 13*5 mm. ; width, 16"0 mm. 



Horizons. — (l) Balclatchie Group ; (2) Stinchar Limestone Group? 



Localities. — (l) Ardmillan, Balclatchie ; (2) Craighead. 



Remarks. — This shell may be provisionally regarded as a variety of 0. testudi- 

 naria, Dalm., but owing to the confusion connected with this species and the well- 

 marked characters of this Ardmillan form, it may be distinguished by a special 

 varietal name. 



The ribbing is coarser and less regular than in 0. girvanensis from the Balclatchie 

 Beds, and the subdivision of the ribs takes place less uniformly, so that they are less 

 equal in size on the margin and have not the fine regular filiform appearance of that 

 species. The interiors of some small brachial valves, probably attributable to this 

 variety, have the two pairs of well-marked subcircular adductor scars and the cardinal 

 process and crura of the same character as in typical 0. testudinaria, Dalm. The 

 ribbing, but not the shape of the shell, recalls O. Rankini, sens, restr. 



