KUNGL. S\'. VET. AKADEMIENS HANDMNGAR. UANl) 41. N:0 4. 



Opi^tIiol)raiicliia. 



Heart with the auricle mostly hehind the ventricle; respwatory organ a ctenidmm, 

 secondary hranchice or none; nervous system, with a few exceptions, of the euthyneurous 

 type; animal always hermaphrodite. 



Synopsis of Families. 



Subordo 1 Tectibranchia. 



Right ctenidum usually present, more or less enclosed in a niantle-cavity ; 

 shell mostly present, external or internal, often rudimentary. 



A. Bulloidea (Cephalaspidea) : Head with the tentacles coalesced to a broad disc; para- 

 podia more or less developed. 



1. Shell external, 



a. with operciilum ; spire elevated ; colnmella with a strong fold ; no para- 

 podial lobes; radula oo.O.oo Fam. Actceonidce. 



b. without operculnm; spire mostly concealed; colimiella with no or only a 

 slight fold. 



+ . Radula absent; no parapodial lobes; in the stomach 3 flat, oval car- 



tilaginous plates with coarsely tuberculated inner faces; cephalic disc 



produced in two erected processes behind Fam. Tornatinidce. 



+ +. Radula present; cephalic disc without produced lobes ; parapodia well 



developed (except in Diaphana). 



a. Radula narrow, 1.1.1 or (2—5) .1.1.1. (2 — 5) ; shell compaet and 

 inflexible, not covered by the parapodia; stomach-plates 3 calca- 

 reous, or none Fam. Scaphandridoe. 



|i. Radula broad, oo . 1 . oo ; shell very thin and flexible; aperture 

 with a posterior slit at the suture; parapodial lobes reflexed över 

 the shell; stomach plates numerous, cartilaginous Fam. Aceridoe. 



2. Shell internal, coasisting of two or three whorls only; aperture large; body 

 oblong ; parapodial lobes more or less folded över the back ; radula without 

 lamina media Fam.. Philinidce. 



