3D0 



Surface marked with comparatively distant, large and prominent, but 

 rather narrow, subangular, more or less acute and slightly flexuous 

 transverse ribs, of unequal length also by a few transverse constrictions, 

 representing periodic arrests of growth. The longer ribs, which extend 

 on both sides to the umbilical margin, often bifurcate at about the middle 

 of each side, but some of them are simple. On the outer volution of the 

 best specimen known to the writer, there are twenty-six of these longer 



Fig. 24. — Pachydiscus muUisulcatus. Outline of transverse section of the outer volution, 



between the ribs and near the aperture, of the specimen from North 



West Bay, V.I., represented on Plate 50. 



ribs, and six or seven periodic constrictions. A single shorter rib is 

 usually intercalated between each pair of the longer ribs, but in one place 

 there are three of the shorter ribs between two of the longer ones. All 

 the ribs are separated by widely concave grooves, and on the periphery of 

 the specimen figured, between the last pair of constrictions, the ribs 

 average from eleven to fourteen millimetres apart, at their summits. The 

 periodic constrictions are not very conspicuous, but are usually a little 

 deeper than the ordinary grooves between the ribs. 



Sutural line unknown, only a small part of the septation being visible 

 in either of the specimens known to the writer. 



