316 MISS MCDONALD AND DE. TRTJEMAN ON [vol. Ixxvii, 



A slender form, whorls nearly flat, and separated by faint 

 sutures ; the whorls preceding the adult whorls are ornamented by 

 three rows of nodides, and resemble somewhat those of P. ogerieni. 

 but the ])osterior row of tubercles is more accentuated than the 

 anterior set. Towards the base of the shell there is a gradual 

 change in tlie ornamentation, the nodules become elongated in the 

 direction of tlie spirals ; there is also a change in the sutural angle, 

 as a result of which the base is attenuated, and the shell becomes 

 2)U])oid. Oral margin circular, no suggestion of a canal. 



.I)e velo))ment. — Tlic apex shows a smooth embryonic whorl, 

 one volution in length ; a primary spiral then appears anteriorly, 



Fig. 8. — Procerithium ('Ex- Fig. 9. — Procerithium ('Ex- 

 elissa "") cf. nuniismale elissa ') ? numismale 

 (Tate). x'O. L.Richard- (Tate), x 5. L. Richard- 

 son coll. L. G. 45. son coll. L. G-. 39. 



and b}^ the succeeding whorl a second spiral is added above the 

 first (and is not so })ron()unced as the first formed). Tliis type of 

 ornamentation is continued for two and a half volutions, after 

 which axials are de\'eloped, producing two rows of nodules which 

 bv the following whorl have become equal in size, owing to the 

 ecpialization in strength of the two spirals. A faint median s})ii'al 

 is then intercalated, producing the three rows of nodules character- 

 istic of the remainder of the shell. As the adult whorls are 

 approached, however, the axial ornament is gradually lost, the 

 spirals (particularly the two anterior) becoming wavy lines. This 

 latter ])eculiarity, along with the change in sutural angle, has been 

 regardi'd hx us as catagenetic. Extdissa icrldonis Hudleston 

 (9, ])1. xi, fig. 8) may be regarded as a morphic equivalent of 

 Rrocerithium (JiJ.rrlissa) cf. numismnlc. 



Locality and horizon. — The specimen described above is 

 from the Lower Lias {Jameson i zone). Bishop's Cleeve Station, 



