BY H. LEIGHTON KESTEVEN. 621 



Family RISELLID^. 



Genera Risella, Gray, and Bissellojjsis, Kesteven. 



Family MODULID^. 



Genera Modulus, Gray, Echindla, Swainson, and Peasiella, 



Kevill. 



Family NASSOPSID^. 



Genus I^assopsis, Smith. 

 Reviewing the superfamily as a whole, its anatomic characters 

 are— 1, a much aborted osphradium; 2, simple branchiae; 3, a 

 rather large devolopment of the mucous gland; 4, small salivary 

 glands; 5, a large buccal mass; and 6, a long and practically 

 uniform radula. To these may be added a general similarity of 

 digestive tract and nervous system, and the absence of an intro- 

 vert. Their habitat has been the indicator which led to the 

 inclusion of these families under one designation. It seemed 

 reasonable to expect that a station such as that occupied by 

 Littorina should have had a modifying effect on some of the organs. 

 It is to this that I would assign the degenerate osphradia and 

 branchiae, and perhaps also the large development of the mucous 

 gland. Living so much out of water, the first of these organs would 

 be of little use to the mollusc, and has accordingly become aborted. 

 In Tectarius nodulosus, Gmel. (which in New South Wales 

 lives beyond the reach of even spray, except in rough weather), 

 the osphradium is so simple that, functionally, it must be almost 

 impotent. The simplicity of the branchia? is, I would suggest, 

 due to the same cause. The large supply of mucus may possibly, 

 by being deposited around the operculum, serve to seal the 

 closure of the shell, and thereby prevent evaporation. 



These three characters are constant throughout the super- 

 family; yet, if the extent of their development or abortion may 

 be accounted for by "environment," we are at once presented 

 with the question, are they characters of systematic importance ? 

 That these characters are the result of environment is a matter 

 3f opinion, but in the present instance they may bear weight as 

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