206 CARBONIFEROUS LAMELLIBRANCHIATA. 



Arca Kittebhauseni, von Koenen, 1879. Neues Jahrbuch fur Min., p. 336, pi. vi, 



fig. 9. 

 Nuculana (Leda) inteemedia, Etheridge, 1885. Brit. Foss., part 1, Palaeoz., 



p. 288. 



Specific Characters. — Shell transversely elongate, lenticular, compressed, and 

 flattened. The most tumid portion of the valves is at the umbo, and even here 

 very slightly convex. The anterior end is well developed, and comprises the 

 greater extent of the valve. The anterior border is regularly rounded, and passes 

 with a gradual curve into the inferior. This is convex anteriorly and then almost 

 straight in the greater part of its extent, but inclined upwards posteriorly. The 

 posterior end is narrowed in a dorso-ventral direction ; its border is bluntly sub- 

 truncate, and small in extent. The superior border has its anterior portion 

 gently convex upwards, and at a much higher level than the posterior, for 

 immediately behind the umbones the valve becomes cut away in a curved 

 line, making the umbones the highest part of the shell, and giving them 

 the appearance of pointing backwards. These are situated a little anterior 

 to the middle line, and are subcentral, small, contiguous, and pointed. The 

 line marking off the escutcheon is very obscure ; but there is a small elongated 

 hollow which lies between the elevated superior edge of the shell and the curved 

 line descending from the umbones, which corresponds to it. 



Interior. — The adductor muscle-scars are very shallow and inconspicuous. 

 There is an internal umbonal vertical ridge, very narrow, almost linear. The 

 hinge is normal as shown by the impressions in a cast. Pallial line faint and 

 entire. 



Exterior. — The surface is almost smooth, but under the microscope is seen to 

 be ornamented with close, regular, concentric striae. 



Dimensions. — Fig. 39, PI. XV, measures : 



Antero-posteriorly . . . .19 mm. 



Dorso-ventrally . . . .8 mm. 



From side to side . . . .4 mm. 



Localities. — Scotland : Upper Limestone series — Orchard Muirfoot Burn, 

 New Cummock, Den, Dairy. Lower Limestone series — Craigenglen, Campsie, 

 Brockley, Leshmahagow. Calciferous Sandstone series — Spymill, Water of 

 Leith. 



Observations. — This species is a flattened and compressed form, and is easily 

 distinguished from all other British forms by its almost central umbones and 

 short, blunt posterior extremity. 



As Mr. Etheridge, jun., points out in his remarks on Leda intermedia (op. cit., 

 p. 347), " of all the published descriptions of Carboniferous Ledse within my 

 reach, L. intermedia approaches nearest to Ij. (Yoldia?) Isevistriata, Meek and 



