98 GASTROPODA. [Aspidobranchia. 



forward and downward from behind it, or if bounded by a callus the 

 latter is truncated or excavated posteriorly. Central tooth of radula 

 broad. 



KEY TO GENERA. 



1. Apex in front of the middle, absorbed by the hole, the latter 



bounded inside by a distinct oval hole-callus, truncated 



behind . . . . . . . . FISSTJRIDEA. 



2. Apex central or post-median, persistent. Anal fissure a 



closed hole at summit . . . . . . . . PUNCTURELLA. 



3. No internal hole-callus or septum, apex not absorbed. 



A. Apex subterminal ; shell minute, subauriform, a short 



slit on upper part of outer lip . . . . . . INCISURA. 



B. A slit-fasciole or band in front, distinctly differentiated 



from the other radiating riblets, extending upward 



from the open anterior slit . . . . - . . EMABGINULA. 



C. No distinctly differentiated slit-fasciole or band ; ends 



of muscle-scar distinctly hooked inward ; shell 

 having radiating ribs and erenulated edge, slit short 

 or none. . . . . . . . . . . SUBEMARGINULA. 



D. No anterior slit or slit-fasciole ; muscle-scar near the 



edge of the shell, its front ends not hooked inward ; 

 shell depressed, oblong, truncated or sinuous in 

 front ; no radiating sculpture ; edge smooth . . SCUTUS. 



Genus 1. INCISURA, Hedley, 1904. 



Incisura, Hedley, Rec. A.M., v, 1904, 91. Type : Scissurella lytteltonensis, 

 E. A. Smith. 



Shell minute, subauriform, smooth. Apex submarginal, few whorls, 

 the last very large ; a short slit on the outer lip above the periphery, 

 bounded by a callus on the inner side. There is no operculum. Teeth 

 of radula similar to those of Emarginula. 



He.dley considers it as a member of the Fissurellidce in which de- 

 velopment has been arrested ; the usual subsequent metamorphoses 

 have not been enacted and the larval characters have persisted in 

 adult life. 



The absence of an operculum removes it from the genus Scissurella, 

 and the characters of the radula, which I have examined, show that 

 it is nearly allied to Emarginula. The radula is asymmetrical. 



1. Incisura lytteltonensis, E. A. Smith, 1894. Plate 6, fig. 15. 



Scissurella lytteltonensis, E. A. Smith, P. Mai. S., i, 57, pi. 7, f. 1, 2. Incisura 

 lytteltonensis, E. A. Smith : Hedley, Bee. A.M., v, 92, f. 18 in text ; 

 Iredale, T.N.Z.L, xl, 381. 



Shell minute, subauriform. There is no sculpture, except fine 

 growth-lines. Colour white, horny, or sometimes roseate. Protoconch 

 not prominent, microscopically delicately radiately ribbed. Whorls "2, 



