ACT^EONID^. 135 



It generally has an entire aperture, but a short basal canal is formed 

 in the RingiculidceJ^ 



Dr. Fischer, from whom we take the above paragraph, divides 

 the Cephalaspidea into families as follows : 



' Operculata Adceonidoe. 



( No radula Tornatinidce. 



( Shell external < ( ScaphandridcB. 



I ( Radula present J Bullidce. 



I 1 Aplustridce. 



^ Inoperculata { [ RingicuRdce. 



{Radula present j Gastropteridce. 

 \ Philinidce. 



No radula Doridlidce. 



This grouping is open to some objections, for it places Actceonidce, 

 one of the least differentiated, primitive families, next in the linear 

 series to ro?'naii?a*c?oe, one of the most divergent ; but until the soft 

 parts of a number of the other types are better known, it will be 

 advisable to retain Fischer's arrangement. In the more ancient, 

 primitive forms the radula is wide, with many rows of similar teeth ; 

 in the divergent groups the radula is often reduced to few longitu- 

 dinal rows (as in tsenioglossate and rhachiglossate Pectinibranchs), 

 and the teeth of each transverse series are dissimilar in form. The 

 shell, originally well coiled, has become degenerate and partially un- 

 coiled in numerous distinct genera. 



Family ACT^ONID^ Fischer. 



Ackeonidce Orbigny (in part). — Meek, Amer. Journ. Science (2), 

 XXXV, p. 84, 1863. — Fischer, Man. de Conch., p. 551. — Conf. 

 BouviER, Ann. Mag. Nat. Hist. (6), xi, p. 441, etc. 



Shell entirely external and capable of containing the entire 

 animal ; spiral, with projecting or depressed spire and moderately 

 numerous whorls, the internal whorl-partitions not absorbed ; surface 

 generally sculptured with spiral punctured grooves. Aperture 

 rounded below, with or without columellar folds. Provided with 

 an operculum. 



Animal having a well-developed head-disk, bearing the sessile 

 «yes, and prolonged in two triangular processes behind ; lateral 

 epipodial lobes not developed ; radula composed of many longitu- 

 dinal rows of teeth, all of the same form. 



