74 PROVISIONAL AID TO THE STUDY OF TASMANIAN MOLLUSCA. 



nervous system, gills with three principal pairs of 

 ganglia. No sexual union, fertilisation being accom- 

 plished by the surrounding water containing the male 

 element. Shell composed of two valves hinged 

 together, but occasionally (Barnea) with smaller 

 supernumerary pieces about the hinge. 



Order Siphonida. (lix. to lxxxix.) 

 Animal with siphons, and mantle margins more or less 

 closed ; comprises most of the marine branchiae, in- 

 cluding a large portion of the old order Dimyaria— 

 _ having two well developed muscular impressions. 



Siphons long, partly or wholly retrac- 

 tile ; the pallial impression upon 

 the inside of valve with a sinus. 



Siphons short, not retractile • pallial 

 impression simple without sinus. 



Sub. order. 



Sinupalliata 

 (lxix. to lxxx.) 



Integripalliata 

 (lxxxi. to lxxxix.) 

 Order Asiphonida (xc.-ci.) 

 No siphons ; pallial impression without sinus ; mantle margin 

 open. 

 Two muscular impressions equally dis- Sub. order. 

 Unci ; mantle margins open or 

 closed behind. 



Two unequal muscular impressions ; 

 posterior impressions Za«/e,anterior 

 impression very small ; frequently 

 inequivalve. 



A single posterior or sub-central adductor 

 muscle and impression only 



Homomyaria 

 (xc. to xciii.) 



Heteromyaria 

 (xciv. to xcvi.) 



Monomyaria 

 (xcvii. to ci.) 



FAMILIES, GENERA AND SUB-GENERA, 

 (vii.) Family MuRiciDiE. 

 Shell spiral ; turriculated with an anterior canal ; the whorls 

 thickened by varices or nodules at each rest period in 

 its growth. 



Genus Murrbx. Linn. (9-11.) 

 Shell ovate or oblong ; spire prominent ; whorls convex 

 crossed by three or more continuous varices ; aperture 

 ending below in a canal, which is generally partly 

 closed. 



