RISSOA. 85 



tion with closely disposed strong spiral flat-topped costella, 

 which are decussated above only by pei-pendicular riblets, 

 or else are broken posteriorly into longitudinal series of 

 tubercles that do not extend to the base of the shell. In 

 the broader and subcancellated variety, the body, as well 

 as the smaller turns, is more or less closely traversed 

 throughout by longitudinal folds, that are surmounted and 

 subtuberculated by the spiral costellse, so as to form a more 

 or less crowded decussation. The number of spiral lines 

 on the penult whorl is almost invariably six ; on the 

 body there are generally twice that number underneath, of 

 which only eight or nine are visible on the dorsal surface. 

 The spire, which is about equal in length to the body, and 

 terminates in a rather small point, is composed of five or 

 five and a-lialf volutions, that are divided by an excavated 

 suture, somewhat quickly increase in length, and are either 

 rounded or merely convex. The body is rather narrow 

 for a species of this section of Missoa, and its basal declina- 

 tion is rounded and gradual ; the penult whorl is sometimes 

 rather large in proportion to the rest. The mouth occuj^ies 

 from two-fifths to nearly three-sevenths of the entire 

 length, is rather small, not patulous, of a nearly ovate 

 shape, and has its throat more or less distinctly crenated 

 by raised sulci. The outer lip is thickened behind, and 

 much rounded at the base. The ordinary length is two 

 lines, and the breadth one line. 



Certain individuals of the sub- cancellated variety of this 

 shell bear so striking a resemblance to calat/ms, as to 

 suggest doubts of their specific individuality, but may 

 usually be distinguished with readiness by the smaller size 

 of their lattice- work. 



As Zetlandica seems to replace crenulata northwards, 

 though both species are companions during a portion of 



