326 GASTROPODA. [Pectinibranchia. 



Shell small, narrowly umbilicated or rimate, turreted, subulate, 

 smooth or usually with spiral riblets ; aperture oval, entire, the peri- 

 stome discontinuous ; lip thin, simple. Operculurn corneous, thin, 

 ear-shaped, paucispiral, with marginal nucleus. 



Only a small number of living forms are known, mostly from the 



/ / 



European seas, and about a dozen fossil species, commencing with 

 the Devonian, are referred to the genus. One species, Aclis costellata, 

 Hutt., has been found in the Pliocene of Wanganui. 



Aclis resembles Turritella in form, but is much smaller and typic- 

 ally umbilicated. 



KEY TO SPECIES. 



A. Whorls convex ; fine axial costse, obsolete on the base, which 



bears minute spiral stria?. Umbilical chink narrow or 



obsolete .. .. .. .. .. .. semireticulata. 



B. Whorls shouldered ; 4 spiral threads on the spire-whorls, 



interstices reticulated by finer axial threads. Perforation 



narrow, open . . . . . . . . . . succinctn. 



1. Aclis semireticulata, Murdoch and Suter, 1906. Plate 16, 

 fig. 5. 



Aclis semireticulata, M. & S., T.N.Z.I., xxxviii, 1905 (190(i), 294, pi. 24, 

 f. 33, 34. 



Shell small, subulate, with a rounded base, sublimate, longitudinally 

 costate, and with a spiral thread on the last whorl. The spiral sculpture 

 consists of some minute strise upon the base, occasionally extending 

 above the periphery, and usually absent on the spire-whorls ; the 

 axial ornamentation formed by small rounded costations, obsolete or 

 absent on the base, and generally very variable, some fairly uniform 

 throughout, others feeble on the last whorl, and others again more 

 or less obsolete on all whorls. Colour light horny, hyaline. Epidermis 

 very thin, glossy, membranaceous, present on a few specimens only. 

 Spire conical, higher than the aperture, with a blunt apex. Protoconch 

 formed by about 2 smooth rounded and vitreous volutions. Whorls 6, 

 regularly increasing, rounded ; base convex. Suture deep, margined 

 above by a minute threadlet, which strengthens upon the later whorls, 

 and at last forms a distinct thread below the periphery upon the 

 body-whorl. Aperture broadly ovate. Outer lip regularly rounded, 

 slightly varicose, the varix usually set a little back from the edge. 

 Columella excavated, mostly a little produced below, forming an 

 angulation of the aperture. Inner lip forming a small callosity upon 

 the parietal wall and the columella. Umbilical chink very narrow or 

 obsolete. Operculum unknown. 



Diameter, 1-7 mm. ; height, 3'3 mm. 



Animal unknown. 



Type in the Dominion Museum, Wellington. 



Hab. Off Great Barrier Island, in 110 fathoms. 



