CLASSIFICATION AND PHYLOGENY 

 OF TECTIBRANCHIATA. 



The Tectibranchiate mollusks have been divided by Dr. Paul 

 Fischer into three main groups, based largely upon external anatomy : 



I. Head bearing a fleshy shield ; tentacles or rhinophores (as such) 



usually wanting ; male organ or its orifice widely separated 

 from the female orifice. CEPHALASPIDEA. 



II. Head without shield, bearing a pair of enrolled, erect rhino- 

 phores, with usually an anterior pair of labial tentacular pro- 

 cesses ; gill dorsal ; male and female orifices widely separated. 



ANASPIDEA. 



III. No head shield ; back protected by a fleshy shield or mantle, 



the gill below it on the right side ; male and female orifices 



contiguous or not widely separated. NOTASPIDEA. 



The families of Cephalaspidea are defined below. The group 



Anaspidea contains but one family, Aplysiidce (see page 59). The 



families of Notaspidea, three in number, are defined on page 170 of 



this volume. 



The numerous families of Cephalaspidea, or shield-headed Tecti- 

 branchs, fall into four well-defined groups or series, of about equal 

 rank, as follows : 



Operculate. 



I. Radula multidentate ; no pleuropodial lobes, Actceonidce, Vol. 

 XV, p. 135. 



Not operculate. 



II. Radula multidentate ; pleuropodial lobes developed. 



a. Head-shield without tentacles ; shell thin, light yellow, 

 brown or green ; aperture frequently with a posterior 

 sinus or slit at the suture, Akeratidce, Vol. XV, p. 350. 

 aa. Head-shield bearing 2 or 4 tentacles ; shell decidedly varie- 

 gated ; no posterior slit, Hydatinida, Vol. XV, p. 385. 



