48 



Posterior valve. — Mucro slightly posterior and much 

 raised, dorsal area broad and smooth, anterior portion longi- 

 tudinally ribbed, similarly to the pleural areas in the median 

 valves. Posterior portion of valve margined by a raised, 

 nodulose rib, the portion of valve immediately behind the 

 mucro smooth, posterior portion decorated by radial ribs 

 broken into two or three nodules by deep concentric sulci, 

 the posterior portion of valve slightly recurved. 



Med/ian valve. — Dorsal area slightly beaked, broad and 

 smooth, lateral areas much raised, composed of two rather 

 flattened, nodulose ribs separated by a broad groove; in 

 valve 2 there is slight evidence of two of these grooves. 

 Pleural area broken into flat, longitudinal ribs, separated by 

 deep grooves, those next the dorsal area only traversing part 

 of the area. I counted ten of these grooves in some valves. 



Girdle. — Clothed with highly-polished, flattish, pebble- 

 like scales, with rounded apices. There is no sign of fluting 

 or ribbing on these scales, but under a high power there is 

 some evidence of parallel scratching. 



Measurements. — The dry specimen measures 20 x 11 mm. 



Habitat. — On the underside of loose limestones buried 

 deeply in sand, in holes in the solid limestone reef at 

 Geraldtoii. 



Juvenile shell. — Measuring 9x5 mm., three longitudinal 

 grooves are present in the pleural area, the nodulose character 

 of the lateral areas is only in evidence in the outer half. A 

 juvenile shell of half this size will therefore have unsculptured 

 pleural and lateral areas, but the latter area will be distinctly 

 raised. The anterior valve is practically without sculpture, 

 with the exception of the very fine decussate pattern that 

 covers the whole shell, probably due to the megalopores. 



Comparisons. — While at first sight this shell, with its 

 polished ivory-like appearance, seems very different from R. 

 tricostalis, Pils., its method of sculpture approaches that form. 

 The ribs in the pleural area are less raised and further apart 

 than is the case in that species, and suggest weather-boarding 

 rather than the narrow well-raised ribs, separated bv deep 

 grooves of R. tricostalis. The lateral areas are divided into 

 two ribs, instead of three, and the nodules are more rounded 

 and flatter. The scales, both in shape and lack of grooving, 

 are. of a different character. R. ver corns, Torr and Ashby, 

 which is somewhat kindred in sculpture, has erect, pointed 

 scales, of the same type as R. jacksonensis, Ashby. Then, 

 again, the habit of this species in adhering to the underside of 

 limestone rocks buried deeply in sand is very distinct from 



